Chin.J.Geochem.(2009)28:340–350 DOI: 10.1007/s11631-009-0340-2

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Petrogenesis of massif-type anorthosite complex, Gruber, Central Dronning Maud Land, East Antarctica: Implications for magma source and evolution Anju Pandey1*, Amit Dharwadkar2, Rasik Ravindra3 , and Andy Milton1

1 National Oceanography Centre, European Way, Southampton, SO14 3ZH, United Kingdom 2 Geological Survey of India, Faridabad, NH31, Haryana, India 3 National Centre for Antarctic & Ocean Research, Goa-403804, India * Corresponding author, E-mail: anju.pandey@noc.soton.ac.uk; anwa1@noc.soton.ac.uk Received December 20, 2008; accepted January 17, 2009 Abstract The origin of anorthosite and associated igneous gabbronorite and ferrodiorite was investigated through detailed study of a typical massif-type anorthosite complex from Gruber, Central Dronning Maud Land, East Antarc-tica. Field observations showed that the Gruber Complex is made up of gabbronorite-anorthosite pluton which was intruded by ferrodiorite dykes. Systematic samples collected from the Gruber Complex revealed significant geo-chemical variations within the region. Four rock types have been identified, based on modal proportions of mineral phases and their geochemistry data. Clinopyroxene-gabbronorite and plagioclase-gabbronorite are the two types of gabbronorite with the dominance of clinopyroxene and plagioclase, respectively. Anorthosite is represented by rocks having predominance of plagioclase with minor clinopyroxene. Ferrodiorite is characterized by modal abundance of orthopyroxene and Fe-Ti oxide. Major and trace element systematics showed that all the four rock types are co-magmatic and are related through fractional crystallization. Based on this study, it is reported that clinopyroxene was the first phase to crystallize followed by plagioclase and then Fe-Ti oxides. Furthermore, trace element composi-tion of the parental melt was calculated using LA-ICPMS analysis of the most primitive, pure clinopyroxene found in the clinopyroxene gabbronorite. Our analyses suggested that the parental melt was similar to that of continental arc basalt and showed signatures of subduction-related metasomatism. Based on mineral chemical and geochemical data, it is interpreted that the parent melt went through changing sequence of crystallization which led to the forma-tion of massive anorthosite. Key words massif-type anorthosite; Gruber; East Antarctica; LA-ICPMS; geochemistry

1 Introduction

The massif-type anorthosites are one of the most enigmatic igneous rock types. Their intriguing fea-tures begin with their composition with almost 90% plagioclase. Temporally, they are restricted to the Pre-cambrian Era and tectonically they favour Proterozoic mobile belts. There are several theories attempting to find explanations for their unique chemical, spatial and temporal preferences but there remains a long-standing debate about most of them (Morse, 1982; Emslie, 1985; Ashwal, 1993; Be´dard, 2001). The most fundamental gap in the knowledge about anor-thositic rocks is an uncertainty regarding the relation-ship between anorthosite and associated mafic rocks.

Be´dard (2001) addressed this issue to investigate if anorthosites and associated mafic cumulates are co-genetic; however there is no consensus on this matter till date. Another contentious issue regarding the mas-sif-type anorthosites is the nature and source of their parent melt. Both mantle-derived tholeiitic, high-Al basaltic parent composition as well as lower conti-nental crust originated jotunitic melts has been pro-posed as a probable source for the massif-type anor-thosites (Emslie, 1994; Longhi et al., 1999; Schiellerup et al., 2000). Therefore, further investiga-tions on these aspects would immensely help in deci-phering some of the key characteristics associated with the massif-type anorthosites.

Reported in this paper is the mineral and bulk chemistry of the rock samples of anorthosite and as-

Chin.J.Geochem.(2009)28:340–350 341

sociated igneous gabbronorite, ferrodiorite from the Gruber region of Central Dronning Maud Land (CDML), East Antarctica. We have analyzed mineral chemical and geochemical data to evaluate the rela-tionship of anorthosite with its associated mafic rocks. Further, to decipher the parent melt composition, we have made in-situ measurements of trace and REE elements of primitive clinopyroxene by laser ablation inductively coupled plasma-mass spectrometry (LA-ICPMS).

The rock samples for this study were collected during Indian Antarctic Expeditions to the Gruber massif-type anorthosite complex, CDML (Reports of

the Indian Antarctic Expeditions, 2003–2004 and 2006–2007). During these expeditions, the Gruber Complex was mapped and systematic samples were obtained. The present study is based on 24 samples, selected from unmetamorphozed, igneous massive anorthosite, gabbronorite and ferrodiorite. The pri-mary objectives of this study are: (1) to decipher the geochemical evolution of the Gruber Complex; (2) to characterize the parent melt composition for this com-plex; and (3) to develop a model for the evolution of the parent melt composition, based on integrated in-terpretation of these results.

Fig. 1. Geological map of the Gruber area. The Gruber Complex can be described as a typical massif-type anorthosite suite. The rock sam-ples for this study were collected during Indian Antarctic Expeditions to the Gruber Complex, CDML, Antarctica. The present study is based on 24 samples, selected from unmetamorphozed, igneous massive anorthosite, gabbronorite and ferrodiorite.

2 Geology

The Gruber massif-type anorthosite complex is located between 8◦–14◦E and 71◦–72◦S in the Wohlthat Mountains of CDML, East Antarctica (Fig. 1). This complex is represented by intrusives ranging in com-position from anorthosite, gabbronorite to ferrodiorite (hereafter referred to as the Gruber Complex). The Gruber Complex is part of the Mozambique Belt which evolved as a result of collision between the East and West Gondwana cratons (Dalziel, 1992, 1997; Stern, 1994; Shackleton, 1996; Grunov et al., 1996). This collisional phase, often termed as the "Pan-African Orogeny", resulted in subduction of the Mozambique Sea (Jacobs, 1999). The Pan-African deformation and metamorphism overprinted large ar-

eas of the older crust up to granulite facies grade in East Antarctica. The Gruber Complex was dated at ~600–610 Ma and was considered to have undergone the metamorphic events of Pan-African Orogeny (Ja-cobs, 1999). The first folding, thrusting, isothermal decompression and metamorphism related with this collisional event were dated at 580–560 Ma using metamorphic zircon dating technique (Jacobs, 1999). This metamorphism took place under me-dium-pressure granulite facies conditions at about 6.8±0.5×108 Pa and 830±20℃ (Markl & Piazolo, 1998). The convergence was succeeded by an exten-sional phase, probably representing orogenic collapse. This event, aged around 530–510 Ma, is characterized by large-scale extensional structures, finally resulting in the post-tectonic intrusion of voluminous granitoids.

342 Chin.J.Geochem.(2009)28:340–350 Metamorphic conditions of this event were at low–pressure granulite facies of 4–5 (×108 Pa) and temperatures of about 640±10℃ (Markl & Piazolo, 1998). Subsequent, extensional shearing gave way for a voluminous charnockite cycle and intrusion of post-tectonic syenite-batholiths at 510 Ma (Mikhalsky et al., 1995). This event marked the final stage of Pan-African metamorphism. It was accompanied by retrogression at pressures of approximately (2–5)×108 Pa and temperatures of 480–580℃ (Markl & Piazolo, 1998).

2.1 Field Geology

The Gruber Complex can be described as a typi-cal massif-type anorthosite suite where an intrusive body of anorthosite is associated with gabbronorite and ferrodiorite (Reports of the Indian Antarctic Ex-pedition, 2003–2004 & 2006–2007). Igneous anortho-site-gabbronorite batholiths invaded the gneissic basement rocks of Gruber and their mutual contacts are gradational in the field. Field disposition supports their coeval and cogenetic nature. On the other hand, ferrodiorite dykes intruded into the anortho-

site-gabbronorite assemblage and have sharp contacts with them, implying the later origin of ferrodiorite intrusives. Generally, the arrangement of intrusives coincides with the pervasive structural trends within CDML. Post-emplacement deformation probably ac-counts for recrystallized features observed in the southern part of the Gruber Complex (Fig. 1).

The anorthosite-gabbronorite unit forms a single intrusive body which varies continuously and concen-trically in the modal proportions of the minerals (Ta-ble 1 and Fig. 1). The main mineral by far is plagio-clase, which varies from ~90%–95% near the centre to ~70% near the margin, except in a few areas where it may fall to 55%–60% within the complex. The rocks can be described as anorthosite (~90% plagioclase) in the centre and as gabbronorite (<90%>65% plagio-clase) over most of the remaining areas of the com-plex, except near the margin, where clinopyrox-ene-gabbronorite (<60% plagioclase) occur in a few places. Alternating mafic and felsic-rich bands define the igneous layering in gabbronorite while anorthosite is generally a massive, coarse grained, plagio-clase-rich rock.

Table 1. Representative major element analyses (wt%) of igneous minerals in rocks of the Gruber Complex

Sample Rock type Mineral Abundance (%) SiO2 TiO2 Al2O3

FeO* MnO MgO CaO Na2O K2O Total Mg# An

Cpx (n= 6)) 30 51.2 0.3 2.3 6.6 0.14 18 20.1 0.66 99.3 0.73 Plag (n= 6 65 53.2 0.05 29.9 0.13 9 7.5 99.8 54.515A

Cpx-GN

Cpx- gabbro-norite Others 5

Cpx (n= 6) 15 51.01 3.2 10.1 0.01 10.1 23.6 1.2 99.9 0.5 Amp (n= 6) 5 41.5 0.7 14.01 10.5 0.14 14.1 11 3.21 0.15 98.2 Plag (n= 6) 75 57.2 3.57 26.01 0.14 9.1 7.01 0.45 99.9 55

19A/23 -GN

Gabbro-norite

Others 5 Plag (n= 6) 90 57.11 25.7 0.15 9.2 6.89 0.52 99.6 55.4 Anortho-

site Others 10 Opx (n= 6) 30 53.01 0.13 1.55 24.3 0.44 20 0.59 0.01 100 Plag (n= 6) 35 57.2 0.03 25.9 0.41 0.17 6.21 8.01 1.1 99 40.5Ilm (n= 6) 25 52.02 0.27 45.1 1.52 0.35 99.3 Ferrodio-

rite Cpx (n= 6) 10 51.9 0.87 1.35 18.1 0 7.1 21.2 0.22 101 0.28

Note: Total iron as Fe2+; Mg# = MgO/MgO+FeO; Cpx. clinopyroxene; Plag. plagiocalse; Ilm. ilmenite; Opx. orthopyroxene. Each mineral data represents the average of 6 analyses.

Ferrodiorite intrusives are common near the

Unter Sea (Fig. 1). In addition, on the eastern most flank of the Gruber Complex, oxide-poor, unmeta-morphozed ferrodiorite exposure was observed (Fig. 1). The outcrops of ferrodiorites visibly intruded into the anorthosite, gabbronorite and preserved igneous characters. Igneous layerings have been well pre-served in ferrodiorite and are defined by the bands rich in Fe-Ti oxides and pyroxene (the majority is hy-persthene with minor clinopyroxene) alternate with plagioclase-rich layers. This rock is characterized by higher modal abundance of the pyroxene, Fe-Ti oxides and relative depletion in plagioclase than their hosts.

3 Analytical methods

Analytical facilities of National Oceanography

Centre, Southampron, U.K. were used for this study. ICPMS with laser ablation, a Thermo Elemental PQ2+ICP-MS coupled to a 4D Engineering (Hanover) Excimer laser, was used for the in-situ trace element abundance measurement of the clinopyroxene using LA-following protocols described elsewhere (Murton et al., 2005). The resulting precision as determined by measuring a reference material is <10% for most ele-ments. The bulk major oxides were analyzed on an X-ray fluorescence spectrometer (XRF) and trace element abundances were analyzed by ICP-MS. Pre-cision for all major oxides is around 1% and for trace elements it is estimated to be 5%. Major element compositions of constituent minerals in the investi-gated rocks were analyzed using EPMA Cameca SX 50. Operating parameters for the microprobe include an acceleration voltage of 15 kV, beam current of 20 nA and a beam size of 1–10 mm. Measuring time was

Chin.J.Geochem.(2009)28:340–350 343

20 seconds. Silicates and oxides were used as stan-dards. Detection limits are typically within the range of 0.02 wt%–0.05 wt%.

4 General petrography and mineral chemis-try

4.1 Petrography

The textures in the massive anorthosite, in the central part of the intrusion, are determined by the cumulus mineral plagioclase. The rocks are coarse-grained, plagioclase crystals are considerably larger than the other main minerals, clinopyroxene and opaque oxides. The plagioclase is remarkably uniform in grain size (10–12 mm) and the shape is tabular in parallel to {010}. Polysynthetic albite twins and Carlsbad twins are common and pericline twins are rare. Furthermore, broken or bent crystals are also relatively rare. All crystals have labradorite cores with a relatively thin normally zoned rim. The plagioclase grains show equilibrium texture. In gabbronorite, cli-nopyroxene occupies the interstices among plagio-clase crystals and tends to be in parallel orientation over distances of 30–50 mm in some gabbronorite samples, forming oikocrysts. The texture is thus a variant of poikilitic, which might be called parallel subophitic. Clinopyroxene is pinkish-brown and pleochroic in the gabbronorite and may show equilib-rium texture with plagioclase or clinopyroxene grains. It is occasionally overgrown by biotite or amphibole. Alkali feldspar occurs in small amounts as a late-stage interstitial mineral and as overgrowth. The feldspars thus form an extensive solidus or hypersolvus series. Clinopyroxene-gabbronorite (hence forth referred to as cpx-gabbronorite) at the outer margin of the intru-sion, is reflected by increased modal abundance of the clinopyroxene. Clinopyroxene is rounded and partly altered and defines alternate bands with feldspar-rich layers. Clinopyroxene-rich layers include amphibole which is generally subhedral and may overgrow cli-nopyroxene in gabbronorite. These rocks also include minor Fe-Ti oxides which are observed to fill the in-terstitial space amongst the mafic phases. The in-ter-granular relationships show that in the cpx-gabbronorite, the plagioclase grains enclose cli-nopyroxene grains. This observation revealed earlier crystallization of clinopyroxene followed by plagio-clase in cpx-gabbronorite. However, in gabbronorite rocks it is common to observe plagioclase grains en-closed in clinopyroxene grains. This indicates that during the formation of gabbronorite, plagioclase was the major mineral to crystallize followed by clinopy-roxene crystallization. A gradual decrease in modal abundance in clinopyroxene from cpx-gabbronorite to gabbronorite and then anorthosite, suggested a com-

mon magmatic evolution of the gabbro-norite-anorthosite rocks.

Ferrodiorite is equigranular, with main phases of orthopyroxene and Fe-Ti oxide showing inclusions of plagioclase, amphibole, clinopyroxene. This rock also shows clear evidence of shearing and foliation of the original igneous layering which is defined by alternate bands of mafic and felsic mineral phases. Petrographic observations confirmed the field evidence of ferrodio-rite intrusive being post-gabbronorite-anorthosite rocks.

4.2 Mineral chemistry

The composition of minerals from different litho-types of the Gruber Complex is given in Table 1. Plagioclase of the massive anorthosite-gabbronorite shows that the anorthite contents are around 54%–55%. This is higher than the An values of pla-gioclase in ferrodiorite rocks which is ~ 40%.

The Mg# values [MgO/(MgO+FeOtotal)] of cli-nopyroxenes vary from 0.73 in the cpx gabbronorite to 0.5 in the gabbronorite. This decrease in Mg# of cli-nopyroxene is coupled with a gradual increase in TiO2 (0.3 wt%–0.7 wt%), Al2O3 (2.2 wt%–2.3 wt%), and Na2O (0.6 wt%–0.12 wt%) from cpx-gabbronorite to gabbronorite. The magmatic amphibole in gabbro-norite is titanian Fe-pargasite (Table 1).

The clinopyroxenes of ferrodiorite rocks show low Mg# of 0.28, which is considerably lower than that of the host gabbronorite. Low Al2O3 (1.55 wt%) and TiO2 (0.13 wt%), and Na2O (0.22 wt%) are cou-pled with moderately high K2O (1.1 wt%). Interstitial Fe-Ti oxides generally consist of ilmenite character-ized by high TiO2 (52.02 wt%), MnO (1.52 wt%) and low Al2O3 (0.27 wt%) and MgO (0.35 wt%) contents.

5 Geochemistry

The major, trace element and REE chemistry of the intrusives clearly reflects their mineralogical as-semblage (Table 2). Anorthosite shows very high Al2O3 (22%–24.66%) and CaO abundances (12.5%–14%). Gabbronorite also shares these charac-teristics with high Al2O3 (within the range of 15.8%–17.8%) and CaO (within the range of 10.1%–11.7%). The cpx-gabbronorite has higher abundances of CaO, MgO and Al2O3 than gabbro-norite and anorthosite. Ferrodiorite rocks are charac-terized by relative depletion in Al2O3 and CaO but enrichment in FeO (~15%–16%) and TiO2 (2.7%–3.5%).

Major element variations were examined against MgO (Figs. 2A–D). On the MgO versus SiO2 diagram (Fig. 2A), cpx-gabbronorite and gabbronorite rocks show a negative linear correlation between these ele-

344 Chin.J.Geochem.(2009)28:340–350 ments. This is an exhibition of clinopyroxene frac-tionation. The higher slope of the line of regression in the cpx-gabbronorite than in gabbronorite (Fig. 2A), suggests a higher rate of clinopyroxene crystallization in the more primitive rock. In this plot, anorthosite and ferrodiorite do not show any differentiation trend, indicating a minor role of clinopyroxene fractionation during this stage (Fig. 2A). In the MgO versus TiO2 diagram (Fig. 2B), cpx-gabbronorite, gabbronorite and anorthosite show a linear common line of differentia-tion. Considering that TiO2 is one of the most incom-patible major elements, this trend suggests that these rocks are co-genetic and share a common trend of magmatic evolution. In these diagrams, cpx-gabbronorite plots as primitive rock and gabbro-norite represents its evolved stage which is in tandem highest Mg# of the clinopyroxenes from the cpx-gabbronorite (Table 1 and Fig. 2B). On MgO versus CaO and Al2O3 plots, both gabbronorite and anorthosite show positive linear trends (Fig. 2C, D), reflecting the major role of plagioclase fractionation during these stages. However, the cpx-gabbronorite shows a positive linear trend in MgO vs. CaO but a negative linear trend in MgO vs. Al2O3 (Fig. 2 C, D). This is direct evidence of the fact that in the

cpx-gabbronorite phase, plagioclase fractionation was not a major phenomenon and clinopyroxene crystalli-sation was a dominant feature in the early stage of magma evolution. This is also supported by the Mg# values of clinopyroxenes, which decrease from cpx-gabbronorite to gabbronorite.

Ferrodiorite samples show negative linear trends with Al2O3 and CaO but a positive linear correlation with TiO2 (Fig. 2B–D). The intrusives have the same relationship with FeO as with TiO2, suggesting the prominence of Fe-Ti oxide fractionation during this stage of magmatic differentiation.

Anorthosite rocks plot as clusters and do not show any trend in the major element variation dia-grams, indicating their cumulus nature.

The REE patterns of the different rock types of the Gruber Complex are generally characterized by LREE-enrichment and nearly flat HREE patterns (Fig. 3). Gradual and systematic increases are noticed in LREE/HREE ratio from cpx-gabbronorite [(La/Yb)N= 5–8] to gabbronorite [(La/Yb)N=7–8.16] to anorthosite [(La/Yb)N=18–38]. Gabbronorite displays a prominent positive Eu anomaly with Eu/Eu* values within the range of 1.8–2.7 in the cpx-gabbronorite and within the range of 1.4–3.35 in gabbronorite. The values of

Table 2. Major (wt%) and trace (×10-6) element data of the investigated rocks from the Gruber Complex

Sample No. CPX-GN CPX-GN CPX-GN CPX-GN CPX-GN GN GN GN GN GN GN GN GN

SiO2 50.11 50.45 51.01 51.79 51.88 52 52.22 52.67 52.91 53.02 53.22 54.31 54.89 TiO2 1.3 1.29 1.2 1.18 1.17 0.9 0.8 0.67 0.51 0.46 0.31 0.2 0.15 Al2O3 14.02 14.45 15.01 15.22 15.55 17.79 17.01 17.44 17.11 16.78 16.34 15.9 15.81 FeO 10.5 9.8 9.2 8.6 8.5 8.5 8.55 8.7 8.67 8.33 9 9.67 9.98 MnO 0.2 0.18 0.12 0.12 0.14 0.01 0.01 0.02 0.05 0.05 0.06 0.09 0.1 MgO 7.2 7.01 6.89 6.66 6.3 6.1 5.89 5.66 5.43 5.1 4.8 4.5 4.11 CaO 14.01 13.8 11.89 11.8 11.6 11.74 11.55 11.35 11.29 11.21 11.01 10.56 10.1 Na2O 1.2 1.7 2.8 3.1 3.2 1.7 1.7 2.2 2.4 2.5 2.66 2.78 3.1 K2O 0.38 0.56 1.01 1.2 1.7 0.9 1.1 1.2 1.8 1.9 2.1 2.2 2.4 P2O5 0.16 0.15 0.12 0.17 0.18 0.02 0.02 0.02 0.03 0.01 0.01 0.01 0.02 Total 99.08 99.39 99.25 99.84 100.22 99.66 99.28 99.6 100.1 99.36 99.51 100.22 100.66

Sr 345 348 352 356 358 455 460 467 498 510 520 529 534 Rb 22 25 28 30 34 37 39 40 45 46 48 48 49 Ba 337 382.11 416.22 461.11 498.12 545.78 65 712 816 917 1027 1118 1198 Nb 3.1 3.8 4.4 4.8 5.4 6.01 7.2 8.01 8.99 10.01 11.03 11.97 12.7 Y 23.76 28.01 31.22 34.22 37.11 40.01 46.12 52.23 58.89 64.02 72.9 77.22 84.02 Zr 41.01 48 54 59 64 69 82.44 91.5 102.1 114.24 126.2 137.33 145.2 Cs 0.4 0.5 0.56 0.61 0.62 0.55 0.56 0.65 0.65 0.68 0.69 0.71 0.722 La 7.26 8.57 9.23 10.1 11.2 12 17.5 24 26.2 29.7 32 33.01 44.3 Ce 12.38 15.2 20 25 30 35 42.2 57.9 62.9 77.1 80.76 81.01 106 Pr 1.51 2.01 2.45 2.77 3.01 3.87 4.88 5.01 7.25 9.25 10.6 11.02 11.9 Nd 7 8 10 12 14 16 19.8 24.5 32.1 41 51.1 52.33 52.82 Sm 2.16 2.22 2.31 2.77 3.04 3.77 4.55 6.06 7.73 9.62 11.2 12.1 12.23 Eu 1.3 1.31 1.39 1.6 1.84 3.7 4.03 4.3 4.58 5.44 5.49 5.54 5.9 Gd 1.7 1.9 2.1 2.7 2.9 3.03 3.45 5.92 6.85 8.82 10.4 12.33 12.39 Tb 0.26 0.3 0.32 0.4 0.43 0.4545 0.533 0.746 1.1661 1.4516 1.591 1.9755 1.9845 Dy 1.31 1.51 1.8 2.05 2.1 2.2 2.7 4.19 5.74 7.32 8.8 10.35 10.38 Ho 0.25 0.29 0.31 0.35 0.4 0.45 0.54 0.81 1.17 1.49 1.76 2.08 2.09 Er 0.7 0.8 0.9 1.02 1.1 1.2 1.35 1.91 2.94 3.74 4.41 5.42 5.44 Tm 0.11 0.12 0.13 0.14 0.16 0.18 0.21 0.28 0.48 0.61 0.7 0.73 0.74 Yb 0.6 0.7 0.8 0.9 1 1.01 1.3 1.82 2.99 3.82 4.3 4.39 4.41 Lu 0.09 0.1 0.12 0.13 0.14 0.15 0.21 0.3 0.47 0.59 0.65 0.65 0.68 Hf 1.95 2.22 2.53 2.77 2.96 3.14 3.78 4.2 4.74 5.29 5.84 6.28 6.91 Ta 0.18 0.21 0.237 0.259 0.281 0.303 0.362 0.403 0.448 0.503 0.555 0.6 0.638 Pb 0.5 0.58 0.657 0.716 0.78 0.84 1 1.11 1.24 1.38 1.53 1.67 1.76 Th 3.5 4 4.5 5 5.4 5.8 7 8.04 9 10.4 11.01 11.89 12.67

(to be continued)

Chin.J.Geochem.(2009)28:340–350 345

Table 2. (continued) Sample No. MA MA MA MA MA MA MA MA FD FD FD

SiO2 54.22 54.45 54.41 55 55 54 54 54.7 48.8 49.1 48.6 TiO2 0.14 0.14 0.14 0.14 0.03 0.02 0.01 0.01 3.5 3.1 2.7 Al2O3 23 22.6 22.2 22 23.6 24.66 24.22 24 12.11 12.66 12.97 FeO 0.6 0.6 1.22 1.89 1.99 1.9 1.98 1.99 16.11 15.8 15.1 MnO 0.22 0.24 0.27 0.28 0.3 0.28 0.22 0.2 0.16 0.15 0.12 MgO 3.3 3.1 2.88 2.77 2.61 2.52 2.2 2.01 4.2 4.1 3.88 CaO 14 13.67 13.55 13.6 13.55 13.45 12.8 12.5 8.22 9.78 10.5 Na2O 3.5 4.4 4.01 3.5 3.4 3.4 4 5 3.2 3.1 2.9 K2O 0.3 0.11 0.2 0.24 0.21 0.19 0.18 0.2 3.1 2.5 2.2 P2O5 0.16 0.16 0.18 0.08 0.03 0.02 0.02 0.03 0.1 0.1 0.1 Total 99.44 99.47 99.06 99.5 100.72 100.44 99.63 100.64 99.5 100.39 99.07

Sr 564 572 570 566 559 573 569 555 187 193 210 Rb 12 12.7 12.2 12 12 12.3 12.27 12.2 18 19.22 20.12 Ba 169 171 178 181 190 197 210 221 61 66 68 Nb 7 6.2 7 6.45 6 5 7.57 7 15 17.88 20.02 Y 5 7.59 7.03 6.51 7 6.6 7 6 99.12 111.8 124.23 Zr 37 39 37.21 37 36.7 36.21 39 37.11 171.22 201.34 227.23 Cs 0.89 0.88 0.88 0.1 0.1 0.1 0.1 0.1 0.871 0.86 0.85 La 9 7.75 4.43 4.6 3.86 2.8 2.63 3.39 80.4 82.9 93.5 Ce 25 18.5 11 10 8.31 6.08 5.82 7.29 191 190 230 Pr 3.35 2.03 1.23 1.01 0.78 0.66 0.54 0.72 26.9 27.1 31.4 Nd 12 8.55 5.14 3.93 2.9 2.28 2.04 2.71 119 118 127 Sm 2.56 1.83 1.16 0.76 0.51 0.45 0.39 0.51 28.4 28.8 27.5 Eu 3.2 1.31 1.26 1.35 1.3 1.09 1.22 1.12 3 2.2 2 Gd 2.66 1.69 1.15 0.58 0.33 0.32 0.27 0.35 28.9 27.6 24.3 Tb 5.075 4.484 3.774 Dy 1.89 1.29 1.1 0.39 0.18 0.2 0.16 0.21 27.8 24.7 21 Ho 0.35 0.25 0.23 0.07 0.03 0.04 0.03 0.03 5.86 4.94 4.14 Er 0.92 0.63 0.59 0.18 0.07 0.08 0.06 0.08 14.8 12.3 10.3 Tm 0.15 0.1 0.09 0.03 0.01 0.01 0.01 0.01 2.4 1.94 1.65 Yb 0.7 0.59 0.52 0.17 0.06 0.07 0.05 0.06 14.5 11.8 10.2 Lu 0.1 0.09 0.09 0.03 0.01 0.01 0.01 0.01 2.14 1.74 1.48 Hf 2.2 2.2 2.41 2.44 2.55 2.4 2.5 2.5 7.96 9.29 10.63 Ta 0 0 0 0 0 0 0 0 0.75 0.88 0.99 Pb 0 0 0 0 0 0 0 0 2.05 2.4 2.76 Th 0 0 0 0 0 0 0 0 14.88 16.34 17.85

Note: CPX-GN. clinopyroxene-gabbronorite; GN. gabbronorite; MA. massive anorthosite; FD. Ferrodiorite.

Eu/Eu* in the massive anorthosite samples range from 6 to 11. Hence, a gradual increase in Eu/Eu* value is observed from cpx-gabbronorite to gabbronorite to anorthosite. This is coupled with a systematic decrease in total REE abundance from cpx-gabbronorite to gabbronorite to anorthosite. Interestingly, ferrodiorite shows a negative Eu anomaly with an almost undif-ferentiated REE pattern (Fig. 3). Additionally, fer-rodiorite has the highest total REE concentrations amongst all rock types of the Gruber Complex.

Overall, the geochemical analyses of the investi-gated rocks clearly suggested that the rocks of the Gruber Complex are the result of fractional crystalli-zation of cumulate minerals. The linear variation of MgO with SiO2, TiO2, Al2O3, and CaO of gabbro-norite and ferrodiorite rocks is consistent with an ig-neous differentiation process controlled by fractional crystallization (Fig. 2A–D). The relatively moderate to low SiO2/Al2O3 ratios and high abundances of in-compatible elements (like REEs) provide strong evi-dence for a cumulus nature of the rocks of the Gruber Complex.

6 Parent melt composition

In the past, various methods were employed to

develop information on the composition and nature of the parent melt of rock suites involving fractional crystallization and cumulation. These include using chilled margins (Wager, 1960; Hoover, 1978), con-temporaneous dykes associated with the intrusions (Brooks and Nielsen, 1978) and intraplutonic quench zones (Tegner et al., 1993). Although relevant in their own way, these methods have some significant limita-tions. They involve an assumption that the selected rock is a genuine representative of the melt from which cumulates of interest have crystallized. In the massif-type anorthosite suites, where associated mafic intrusives have experienced fractional crystallization and massive anorthosite represents cumulates, it is difficult to find a true representative of the melt. However, due to current advances in spot analysis technique, ‘in-situ’ approach of minerals together with distribution coefficients can be used to calculate the equilibrium melt composition. It has been shown in several recent studies, involving fractionally crystal-lized mafic and cumulate rocks, that the composition of the parent melt can be estimated from ‘in-situ’ analysis of trace and REE concentrations in the primi-tive clinopyroxene (Hermann et al., 2001; Greene et al., 2006; Jagoutz et al., 2007). For selecting the most primitive rock of the Gruber Complex, we ruled out

346 Chin.J.Geochem.(2009)28:340–350 the possibility that anorthosite represents cumulates and ferrodiorite because they are post-anorthosite phase―the most evolved stage of intrusives. So, for source characterization, we took gabbronorite samples into consideration. Major element variation diagrams show that the cpx-gabbronorite plots at the origin of the line of magmatic differentiation (Fig. 2A–D). REE plots show a decrease in total REE values from cpx-gabbronorite to gabbronorite to anorthosite (Fig. 3). These geochemical observations suggest primitive nature of the cpx-gabbronorite in the intrusive body of gabbronorite-anorthosite. To select the most primitive sample out of the cpx-gabbronorite, we used mineral chemistry data. The cpx-gabbronorite which hosts clinopyroxenes with the highest Mg# is identified as the ‘most primitive’ rock of the suite. These clinopy-roxenes of Mg# ~0.73 from the cpx-gabbronorite were selected for the in-situ LA-ICPMS analysis. The par-ent melt composition was calculated using the clino-pyroxene/melt partitioning from published literature (Hart and Dunn, 1993), as is given in Table 3.

Fig. 2A–D. Selected major element variation diagrams; from top to bottom (A–D) MgO versus SiO2, TiO2, Al2O3 and CaO, respectively.

The trace element pattern of the parent melt is similar to that of gabbronorite samples on a primitive mantle-normalized diagram (Fig. 4A). Interestingly, it shows a total REE abundance value which is lower than that in the ferrodiorite but highest in the gabbro-norite-anorthosite rock samples (Fig. 4A). The parent melt does not show any Eu anomaly. However, it shows depletion in Zr and Nb and enrichment in Sr and Eu. To characterize the chemical nature of the parent melt, we used the Th-Hf-Nb diagram (Wood, 1980) (Fig. 4B). This plot shows that the general characteristics of the parent melt as well as the gab-bronorite intrusives overlap with those of the conti-nental arc basalt.

Fig. 3. Chondrite-normalized REE diagram. Black. Anorthosite; red. gabbronorite; green. ferrodiorite. Chondrite-normalized values are from McDonough and Sun (1995).

Table 3. Parent melt calculation on the basis of primitive

clinopyroxene composition Element Primary Cpx (n=6) Cpx/melt Parent melt

Sr 49.44 0.096 515 Rb 0.0198 0.0006 33 Ba 0.2958 0.00068 435 Nb 0.0588 0.007 8.4 Y 15.7875 0.421 37.5 Zr 13.824 0.128 108 La 1.005 0.05 20.1 Ce 3.852 0.09 42.8 Pr 0.9044 0.14 6.46 Nd 6.688 0.22 30.4 Sm 2.4222 0.33 7.34 Eu 1.0582 0.37 2.86 Gd 3.198 0.41 7.8 Tb 0.5043 0.41 1.23 Dy 3.0914 0.41 7.54 Er 1.8614 0.41 4.54 Tm 0.2542 0.41 0.62 Yb 1.6277 0.41 3.97 Lu 0.2583 0.41 0.63 Hf 0.667 0.23 2.9 Th 0.02226 0.003 7.42

Note: The average of 6 analyses and published cpx/melt data (Hart and dunn, 1993); the concentrations of elements are in (×10-6).

7 Discussion and conclusions

7.1 Magmatic evolution of the Gruber Complex

Chin.J.Geochem.(2009)28:340–350 347

Variations in major element mineral chemistry for the gabbronorite-anorthosite and ferrodiorite intru-sives of the Gruber Complex are consistent with an igneous differentiation process controlled by frac-tional crystallization. Textural evidence indicates the early crystallization of chiefly, clinopyroxene in the cpx-gabbronorite, followed by clinopyrox-enes+plagioclase in the gabbronorite to plagioclase in the anorthosite. This was followed by plagioclase-free ferrodiorite crystallization and intrusion into the ex-isting gabbronorite-anorthosite magmatic body. The crystallization order and mineral chemical variation are reflected in the bulk geochemical analyses of these rocks. This interpretation is corroborated with geo-chemical observations.

Fig. 4. A. Primitive mantle (PM)-normalized diagram of parent melt and gabbronorite. Red. Parent melt; black. gabbronorite. Normaliza-tion values for PM are from Sun and McDonough (1989). B. Ternary plots of Th-Hf/3-Nb/16 showing the discrimination diagram after Wood (1980). IAT. Island arc tholeiite; CAB. calc-alkaline basalt; WPA. within plate alkaline basalt; MORB. mid-ocean ridge basalt; E-MORB. enriched type MORB; N-MORB. normal type MORB; WPT. within plate tholeiite.

The variation of major elements against MgO

shows a common line of magmatic differentiation between cpx-gabbronorite and gabbronorite, which

suggests their cogenetic relationship. The disposition of cpx-gabbronorite in these plots indicates its primi-tive nature which is supported by high Mg# values of the clinopyroxenes analyzed from this rock type. Similar REE patterns of the rocks with a gradual in-crease in Eu anomaly from cpx-gabbronorite to gab-bronorite to anorthosite indicate their in-ter-relationship through fractional crystallization. Field and petrographic observations also support co-genetic relationship between gabbronorites and anorthosite. Ferrodiorite rocks show similar REE pat-terns to those of the rest of the investigated rocks, but with negative Eu anomalies. Field and petrographic observations also suggest that ferrodiorite intrusion was subsequent to the gabbronorite-anorthosite intru-sive phase. However, to unambiguously decipher the genetic relationship amongst the intrusives of the Gruber Complex, we applied the established incom-patible ratio plot of Zr/Nb versus Y/Nb. These ele-ments are highly incompatible in general and in par-ticular the minerals involved in the rocks studied. Hence, their ratios are most likely to be unaffected by the magmatic differentiation process of the investi-gated rocks. In this figure, cpx-gabbronorite, gabbro-norite and ferrodiorite rocks share a common line of regression (Fig. 5). This suggests that intrusives of ferrodiorite and gabbronorite-anorthosite are geneti-cally related.

Fig. 5. Zr/Nb vs. Y/Nb diagram of the investigated gabbro-norite-anorthosite and ferrodiorite rocks. Similar incompatible ele-ment ratios indicate their cogenetic origin.

7.2 Parent melt and source characterization

The possession of negative Nb, Zr and positive Sr, Eu anomalies in the parent melt, along with continen-tal arc basalt signatures, suggests that the source of the Gruber anorthosite is related to the subduction-type setting. On the Nb/Ba vs. Nb/Zr diagram (Hooper and Hawkesworth, 1993), the parent melt as well as gab-bronorite appears to have an overlap with the sub-continental lithospheric mantle (Fig. 6A). Both the parent melt and gabbronorites have Nb/La ratios less than 1, which can be interpreted to have been de-rived from a sub continental lithospheric mantle (SCLM) altered by subduction-related metasomatism.

348 Chin.J.Geochem.(2009)28:340–350 Hence, the parent melt and gabbronorite were evalu-ated for geochemical features associated with subduc-tion-related mantle metasomatism in SCLM. This is specifically demonstrated in the La/Nb vs. La/Ba dia-gram (Fig. 6B). All the samples of gabbronorite and the parent melt composition fall in the subduction modified continental lithospheric mantle. Sun and Stern (2001) used Ba/La, Pb/Ce, Cs/Rb and U/Th ra-tios to identify fluid-enriched sources for different types of mantle metasomatism. In these ratios, the numerator has higher fluid mobility than the denomi-nator. This means that the magmas derived from

fluid-enriched sources would be enriched in Ba, Pb, Cs and U relative to La, Ce, Rb and Th, respectively. If the mantle is enriched by melts from subducted sediments, it is reflected in Th/Yb ratio in mafic magmas. In the present study, the Th/Yb versus Ba/La diagram (Fig. 6C) shows gabbronorite and the parent melt are enriched in Ba, as well as in Th. This sug-gests enrichment of the melt by both fluid-dominated metasomatism and sediment melting. Hence, it is in-ferred that the source of the Gruber Complex was its SCLM which was modified by subduction zone fluid as well as sediment-derived melt.

Fig. 6. A. Nb/Ba vs. Nb/Zr; B. La/Nb vs. La/Ba; C. Th/Yb vs. Ba/La. In A, parent melt composition overlaps with SCLM; in B parent melt and gabbronorite fall within the field of subduction modified SCLM; in C vertical vector indicates slab-derived fluids; horizontal vector point towards sediment derived enrichment. Composition of SCLM from Rudnick and Gao (2003).

7.3 Parent melt to massive anortho-site-crystallization sequence

The parent melt which originated from SCLM led to the formation of gabbronorite-anorthosite body through changing sequence of crystallization of min-erals. Petrographic observations of clinopyroxene in-clusions in plagioclase indicate that clinopyroxene fractionation took place before plagioclase in the cpx-gabbronorite. This explains the lack of a signifi-cant Eu anomaly in the parent melt and the cpx-gabbronorite whole rock geochemistry (Fig. 4A, B). However, frequent inclusions of plagioclase within clinopyroxenes imply crystallization of plagioclase prior to clinopyroxenes in the gabbronorite. The REE concentrations of cpx-gabbronorite to gabbronorite

samples show that the increases in incompatible ele-ment concentrations and Eu anomaly are associated with continuously decreasing Mg# of clinopyroxenes (Fig. 3 and Table 2). This clearly suggests that the se-quence of crystallization was from cpx to plagioclase in the more mafic cpx-gabbronorite. However, it shifted from plagioclase to cpx in gabbronorite and finally to only plagioclase crystallization in anortho-site. This change in crystallization sequence can be explained by compositional (H2O and/or Na contents) and pressure effects (Elthon & Scarfe, 1984; Gust & Perfit, 1987; Bartels et al., 1991). Experimental pe-trology studies indicated that the crystallization se-quence, especially of plagioclase and clinopyroxene, would depend on the ambient pressure and the water content of the system (e.g. Gaetani et al., 1993; Sisson & Grove, 1993; Panjasawatwong et al., 1995; Villiger

Chin.J.Geochem.(2009)28:340–350 349

et al., 2007). At 1–2 GPa, in the presence of H2O, crystallization of plagioclase was suppressed and cli-nopyroxene fractionated prior to plagioclase (Muen-tener et al., 2001). At lower pressure (0–2 GPa), even in the presence of water, plagioclase would crystallize prior to clinopyroxene (Sisson & Grove, 1993). We proposed that the cpx-gabbronorite sequence crystal-lized at higher pressure (1–2 GPa) and gabbronorite crystallized at lower pressure around 0.5–0.7 GPa. Finally, due to complete fractionation of clinopyrox-ene, the melt became enriched in plagioclase and this led to in-situ crystallization of massive anorthosite at the same or shallower levels. Exposure of the two rock units of gabbronorite and anorthosite, at the same structural level, without tectonic discontinuity, sup-ports diapirism of the anorthosite into the unconsoli-dated gabbronorite. The residual melt, enriched in Fe-Ti oxide, intruded into the gabbronorite-anorthosite pluton at a later stage, is probably related to continen-tal scale shearing.

8 Conclusions

The gabbronorite-anorthosite and ferrodiorite rocks of the Gruber Complex crystallized from a common, hydrous and mantle-derived magma. Min-eral chemistry and geochemistry data indicate that the cpx-gabbronorite rocks crystallized at higher pressure than the gabbronorite-anorthosite sequence. Petrological and chemical data further indicate a dif-ference in crystallization mechanism. The gabbro-norite rocks were dominantly formed from fractional crystallization and melt-rock reaction between 2–0.7 GPa, whereas the anorthosite sequence was domi-nantly formed from in-situ crystallization at 0.5–0.7 GPa. Exposure of the two rock units at the same structural level, without tectonic discontinuity, sup-ports diapirism of the anorthosite rocks into the un-consolidated. The new data presented here imply that percolative fractional crystallization and magmatic differentiation are the dominant fractionation mecha-nisms in the upper mantle from which the Gruber magma came. Within the lower crust, however, the already fractionated liquid evolved at lower pressure through in-situ crystallization.

Acknowledgements The authors gratefully ac-knowledge Dr. S. Mookherjee, Director of Geological Survey of India for his help during the course of this study. RR and AD are thankful to fellow team mem-bers during Indian Antarctic Expeditions to Gruber (2003–2004 and 2006–2007). AP is thankful to the University of Southampton and National Oceanogra-phy Centre, Southampton, UK for post-doctoral fund-ing. Cooperation of the geochemistry division of the NOC, Southampton, is sincerely acknowledged.

Valuable comments and suggestions from both the reviewers helped significantly in revising the manu-script.

References

Ashwal L.D. (1993) Anorthosite [M]. pp.422. Springer, Berlin Heidelberg, New York.

Bartels K.S., Kinzler R.J., and Grove T.L. (1991) High pressure phase rela-tions of primitive high-alumina basalts from Medicine Lake Volcano, Northern California [J]. Contrib. Mineral and Petrol. 108, 253–270.

Be´dard J.H. (2001) Parental magmas of the Nain Plutonic Suite anortho-sites and mafic cumulates: A trace element modelling approach [J]. Contrib. Mineral. Petrol. 141, 747–771.

Brooks C.K. and Nielsen T.F.D. (1978) Early stages of the Skaergaard magma as revealed by a closely related suite of dike rocks [J]. Lithos. 11, 1–14.

Dalziel I.W.D. (1997) Neoproterozoic-Paleozoic geography and tectonics: Review, hypothesis, environmental speculation [J]. Geol. Soc. Amer-ica Bull. 109, 16–42.

Dalziel I.W.D. (1992) Antarctica: A tale of two supercontinents?[J]. Ann. Rev. Earth. Planet. Sci. 20, 501–526.

Elthon D. and Scarfe C.M. (1984) High-pressure phase equilibria of a high-magnesia basalt and the genesis of primary oceanic basalts [J]. American Mineralogist. 69(1–2), 1–15.

Emslie R.F. (1994) Petrogenesis of a Mid-Proterozoic anortho-site-mangerite-charnokite granite (AMCG) complex: Isotopic and chemical evidence from the Nain plutonic complex [J]. J. Geol. 102, 539–558.

Emslie R.F. (1985) Proterozoic anorthosite massifs. In The Deep Crust in the North Atlantic Provinces, NATO Advanced Study Institute Series C 158 (eds. Tobi A.C. and Touret J.R.L.) [M]. pp.39–60. Reidel, Dordrecht.

Gaetani G.A., Grove T.L., and Bryan W.B. (1993) The influence of water on the petrogenesis of subduction-related igneous rocks [J]. Nature. 365, 332–334.

Greene A.R., DeBari S.M., Kelemen P.B., Blusztajn J., and Clift P.D. (2006) A detailed geochemical study of the island arc crust: The Talkeetna arc section, south-central Alaska [J]. Jour. of Petrology, 47, 1051–1093.

Grunov A., Hanson R., and Wilson T. (1996) Were aspects of Pan- African deformation linked to Iapetus opening? [J]. Geology. 24, 1063–1066.

Gust D.A. and Perfit M.R. (1987) Phase relations of a high-Mg basalt from the Aleutian Island Arc: Implications for primary island arc basalts and high-Al basalts [J]. Contrib. Mineral and Petrol. 97, 7–18.

Hart S.R. and Dunn T. (1993) Experimental CPX/ Melt partitioning of 24 trace elements [J]. Contrib. Mineral. Petrol. 113, 1–8.

Hermann J., Muentener O., and Guenther D. (2001) Differentiation of mafic magma in a continental crust-to-mantle transition zone [J]. Jour. of Petrol. 42, 189–206.

Hooper P.R. and Hawkesworth C.J. (1993) Isotopic and geochemical con-straints on the origin and evolution of the Columbia River Basalts [J]. Jour. Petrol. 34, 1203–1246.

Hoover J.D. (1978) Melting relations of a new chilled margin sample from the Skaergaard intrusion [J]. Carnegie Inst. Wash. Yearb. 77, 739–743.

Indian Antarctic Expedition Report (2003–2004) National Centre for Ant-arctic & Ocean Research, Goa, 403804, India.

350 Chin.J.Geochem.(2009)28:340–350 Indian Antarctic Expedition Report (2006–2007), National Centre for Ant-

arctic & Ocean Research, Goa, 403804, India Jacobs J. (1999) Neoproterozoic/lower Palaeozoic events in central Dron-

ning Maud Land (East Antarctica) [J]. Gondwana Research. 2, 473–480.

Jagoutz O., Muentener O., Ulmer P., Pettke T., Burg J-P., Dawood H., and Hussain S. (2007) Petrology and mineral chemistry of the lower crustal intrusions: The Chilas Complex, Kohistan (NW Pakistan) [J]. Jour. Petrol. doi:10.1093/petrology/egm044. 1–59.

Longhi J., Van der Auwera J., Fram M.S., and Duchesne J-C. (1999) Some phase equilibrium constraints on the origin of Proterozoic (massif) anorthosites and related rocks [J]. J. Petrol. 40, 339–362.

Markl G. and Piazolo S. (1998) Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land, Antarctica; monitors of fluid compositional changes during late-magmatic rock-rock interac-tion processes [J]. Contrib. Mineral and Petrol. 132, 246–268.

McDonough W.F. and Sun S.S. (1995) The composition of the Earth [J]. Chem. Geol. 120, 23–253.

Mikhalsky E.V., Beliatsky B.V., Savva E.V., Wetzel H.-U., Federov L.V., Weiser T., and Hahne K. (1995) Reconnaissance geochronologic data on polymetamorphic and igneous rocks of the Humboldt Mountains, Central Queen Maud Land, East Antarctica. In The Antarctic Region: Geological Evolution and Processes. Proceedings of the VII Interna-tional Symposium on Antarctic Earth Sciences, Siena (ed. Alberto R.C) [M]. pp.45–54. Terra Antarctica Publication, Siena.

Morse S.A. (1982) A partisan review of Proterozoic Anorthosites [J]. Am. Mineral. 67, 1087–1100.

Muentener O., Hermann J., and Trommsdorff V. (2001). Cooling history and exhumation of lower-crustal granulite and upper mantle (Malenco, eastern Central Alps) [J]. Jour. of Petrol. 41, 175–200.

Murton B.J., Tindle A.G., Milton J.A., and Sauter D. (2005) Heterogeneity in southern central Indian ridge MORB: Implications for ridge hotspot interaction [J]. Geochemistry, Geophysics, Geosystems. 6, Q03E20, doi:10.1029/2004GC000798.

Panjasawatwong Y., Danyushevsky L.V., Crawford A.J., and Harris K.L. (1995) An experimental study of the effects of melt composition on plagioclase; melt equilibria at 5 and 10 kbar; implications for the ori-gin of magmatic high-An plagioclase [J]. Contributions to Mineralogy

and Petrology. 118, 420–432. Rudnick R.L. and Gao S. (2003) Composition of the Continental Crust. In

The Crust (ed. Rudnick R.L.) 3, 1–64, Treatise on Geochemistry (eds. Holland H.D. and Turekian K.K.) [M]. Elsevier-Pergamon, Oxford.

Schiellerup H., Lambert R.D., Prestvik T., Robins B., McBride J., and Lar-sen R.B. (2000) Re-Os isotopic evidence for a lower crustal origin of massif-type anorthosites [J]. Nature. 405, 781–784.

Shackleton R.M. (1996) The final collision zone between East and West Gondwana: Where is it? [J]. Jour. Afr. Earth Sci. 23, 271–287.

Sisson T.W. and GroveT.L. (1993) Experimental investigations of the role of H2O in calc alkaline differentiation and subduction zone magmatism [J]. Contrib. Mineral. Petrol. 113, 143–166.

Stern R.J. (1994) Arc assembly and continental collision in the Neoprotero-zoic East African Orogen: Implication for the consolidation of Gond-wanaland [J]. Ann. Rev. Earth Planet. Sci. 22, 319–351.

Sun S.S. and McDonough W.F. (1989) Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. In Magmatism in Ocean Basins (Special Publication No. 42) (eds. Saun-ders A.D. and Norry M.J.) [M]. pp.313–345. Geological Society of London.

Sun C.H. and Stern R.J. (2001) Genesis of Mariana shoshonites: contribu-tion of the subduction component [J]. J. Geophys. Res. 106 (B1), 589–608.

Tegner C., Wilson J.R., and Brooks C.K. (1993) Intraplutonic quench zones in the Kap Edvard Holm layered gabbro complex, East Greenland [J]. Jour. Petrol. 34, 681–710.

Villiger S., Ulmer P., and Muentener O. (2007) Equilibrium and fractional crystallization experiments at 0.7 GPa–The effect of pressure on phase relations, liquid compositions and mineral-liquid exchange reactions of tholeiitic magmas [J]. Jour. of Petrol. 48, 159–184.

Wager L.R. (1960) The major element variation of the layered series of the Skaergaard Intrusion and a re-estimation of the average composition of the hidden layered series and of the successive residual magmas [J]. J. Petrol. 1, 364–398.

Wood D.A. (1980) The application of a Th-Hf-Ta diagram to problems for tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province [J]. Earth and Planetary Science Letters. 50, 11–30.