Contrib Mineral Petrol (2006) 152:591–609 DOI 10.1007/s00410-006-0122-3
ORIGINAL PAPER
Mineralogy and geochemistry of microgranular enclaves in Palaeoproterozoic Malanjkhand granitoids, central India: evidence of magma mixing, mingling, and chemical equilibration Santosh Kumar Æ Vikoleno Rino
Received: 28 December 2004 / Accepted: 16 June 2006 / Published online: 22 August 2006 Springer-Verlag 2006
Abstract Palaeoproterozoic (ca 2,480 Ma) felsic magmatism of Malanjkhand region of central Indian Precambrian shield, referred to as Malanjkhand granitoids (MG), contain xenoliths of country rocks and mesocratic to melanocratic, fine-grained porphyritic microgranular enclaves (ME). The shape of ME is spheroidal, ellipsoidal, discoidal, elongated, and lenticular, varying in size from a few centimeters to about 2 m across. The contact of ME with the host MG is commonly sharp, crenulate, and occasionally diffuse, which we attribute to the undercooling and disaggregation of ME globules within the cooler host MG. The ME as well as MG show hypidiomorphic texture with common mineral Hbl-Bt-Kfs-Pl-Qtz assemblage, but differ in modal proportions. The variation in minerals’ composition, presence of apatite needles, elongated biotites, resorbed plagiclase, ocellar quartz, and other mafic–felsic xenocrysts strongly oppose the restite and cognate origins of ME. Compositions of plagioclases (An3–An29), amphiboles (Mg/Mg+Fe2+=0.55–0.69), and biotites (Mg/Mg+Fe2+=0.46–0.60) of ME are slightly distinct or similar to those of MG, which suggest partial to complete equilibration during mafic– felsic magma interactions. Al-in-amphibole estimates
Communicated by T.L. Grove S. Kumar (&) Department of Geology, Kumaun University, Nainital 263 002, Uttaranchal, India e-mail:
[email protected] V. Rino Department of Geology, Nagaland University, Kohima 797 002, Nagaland, India e-mail:
[email protected]
the MG pluton emplacement at ca 3.4 ± 0.5 kbar, and therefore, magma mixing and mingling must have occurred at or below this level. The Fe Mg substitution in biotites of ME and MG largely suggests subduction-related, calc–alkaline metaluminous (I-type) nature of felsic melts. Most major and trace elements against SiO2 produce near linear variation trends for ME and MG, probably generated by the mixing of mafic and felsic magmas in various proportions. Trace including rare earth elements patterns of ME–MG pairs, however, show partial to complete equilibration, most likely governed by different degrees of elemental diffusion. The available evidence supports the model of ME origin that coeval mafic (enclave) and felsic (MG) magmas produced a hybrid (ME) magma layer, which injected into cooler, partly crystalline MG, and dispersed, mingled, and undercooled as ME globules in a convectively dynamic magma chamber.
Introduction Microgranular enclaves (ME) are commonly reported from volcanic (e.g., Eichelberger 1980; Bacon and Metz 1984; Bacon 1986; Davidson et al. 1990; Clynne 1999; Eichelberger et al. 2000; Kus¸ cu and Floyd 2001; Ban et al. 2005) and plutonic (e.g., Didier 1973; Reid et al. 1983; Cantagrel et al. 1984; Vernon 1984; Petrı´k and Broska 1989; Castro 1990; Dorais et al. 1990; Didier and Barbarin 1991; Kumar 1995; Mass et al. 1997; Wiebe et al. 1997; Silva et al. 2000; Waight et al. 2000, 2001) igneous suites. The ME hosted in granitoids may serve as a potential tool to understand the process of mafic and felsic magma interaction, where
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ME can represent pristine mafic magma or a hybrid (mafic and felsic) product mingled in convective, calc– alkaline plutonic systems (Castro et al. 1990; Wiebe et al. 1997). In some cases, the ME are considered as fragments of recrystallized, refractory and residual materials derived from the granite source region (Chappell et al. 1987; Chen et al. 1989), and in some cognate (cumulate) fragments of felsic host (Dodge and Kistler 1990). Although the origin of ME is still debated, the most popular choice is the magma mixing and mingling models. Magma mixing causes homogenization of the interacting melt phases and the conversion of early crystals to partly dissolved (corroded) in new hybrid magma, whereas, mingling or co-mingling involves partial mixing or interpenetration of felsic–mafic magmas without pervasive changes (e.g., Vernon 1983; Sparks and Marshall 1986; Barbarin and Didier 1992). The ME, therefore, must bear some field, petrographic, mineralogical, and geochemical features relevant to operative magmatic processes such as magma mixing, autolith, restite-melt separation etc. The felsic magmatism in the Malanjkhand region of central India is represented by Cu(± Mo ± Au) hosting Palaeoproterozoic granitoids referred herewith as Malanjkhand granitoids (MG), which contain numerous enclaves such as country rock xenoliths and ME. The MG have been considered metaluminous to peraluminous, calc–alkaline, porphyry copper-bearing felsic igneous plutons formed by the partial melting of deep-seated crust (Rai and Venkatesh 1990, 1993) and evolved in a volcanic arc tectonic setting (Sikka and Nehru 1997). Whole rock Rb-Sr ages (2,362 ± 58, 2,467 ± 38, 2,243 ± 217 Ma) of MG have been provided (Ghosh et al. 1986; Panigrahi et al. 1993). New SHRIMP RG data on zircon from MG suggest the age of granitic activity at ca 2,480 Ma, and hence whole rock Rb-Sr age (ca 2,400 Ma) can be attributed to hydrothermal overprint (Panigrahi et al. 2004). Re-Os ages of molybdenite show that mineralizations in MG were almost contemporaneous at 2,490 ± 8 Ma (Stein et al. 2004). Numerous studies have been carried out on the origin of Cu-Mo-Au deposit, but no attention has been focused on the origin of ME hosted in MG. Recently, detail field feature and major elements geochemistry of ME and MG have been described (Kumar et al. 2004a, b). In this paper, we provide a comprehensive petrography, mineral chemistry, major and trace including rare earth elements geochemistry of ME and host MG, and discuss the possible origin of ME and their degree of chemical equilibrium with respect to felsic host MG.
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Contrib Mineral Petrol (2006) 152:591–609
Geology, field relation, and petrography The MG cover an area of about 1,500 sq km in the Balaghat district of Madhya Pradesh, central India, representing one of the Proterozoic felsic magma systems associated with the central Indian suture (CIS) zone (Yedekar et al. 1990). The central Indian craton, being an integral part of the Precambrian shield of India, is considered a collage of two ancient cratons viz. southern Indian craton and northern Indian craton that probably amalgamated and stabilized at ca 2.5 Ga along the Central Indian Tectonic Zone (Acharyya and Roy 2000; Stein et al. 2004) (Fig. 1a). The MG represent a shallow level pluton (~3.5 kbar) and are associated with the Nandgaon Group of bimodal volcano-sedimentary lithounits (Ramachandra and Rao 1998). The MG are overlain by the Chilpi metasediments in the southeastern and western parts of the pluton (Fig.1b). The older Amgaon schist and gneiss are exposed in the northwest and west of the pluton, respectively, whereas the Nandgaon volcanics are exposed in the southern and eastern margins of the pluton. In the northeastern part of the MG pluton, the Deccan volcanics are exposed. The MG are intruded by quartz veins and basic dykes. Coarse-grained equigranular, porphyritic, and fine-grained varieties of MG can be recognized. The ME are ubiquitous in coarsegrained equigranular and porphyritic varieties of MG. Xenoliths of older country rocks exhibit angular shape, sharp but reactive contact and metamorphic foliation, occurring frequently near marginal parts of the MG pluton, and therefore, appear caught up at the emplacement level. Based on microstructures and mineralogy, the ME can be classified into three major types: (1) mesocratic to melanocratic, coarse- to medium-grained ME with phenocrysts of plagioclase (up to 1 cm), amphibole (up to 1.5 cm), and biotite (
