Earth Science India Vol.1 (IV),October, 2008, pp. 204-219

http://www.earthscienceindia.info/

Granite series evaluation of Early Ordovician Kyrdem granitoids and enclaves, Meghalaya plateau, northeast India: implication on oxidation condition of interacting mafic-felsic magma system

Santosh Kumar* and Kh. Mohon Singh**
*Department of Geology, Kumaun University, Nainital 263 002, India
**Department of Geology, Imphal College, Imphal, 795 001, India

Abstract: Early Ordovician (479±26 Ma) Kyrdem granitoids (KG) and their enclaves in Meghalaya plateau have been evaluated in terms of magnetic susceptibility (MS) and whole rock composition in order to understand the redox condition of comingled mafic and felsic melts. KG (MS=15.56-31.55×10^-3SI) and microgranular enclaves ME (MS=24.47-32.09×10^-3SI) are moderately to strongly oxidized, magnetite series granites formed in a late- to post-tectonic calc-alkaline, mafic-felsic magma interacting environment. Oxidizing nature of bulk KG melt was locally reduced to ilmenite series (0.26×10^-3 SI) granite near the margin of the KG pluton as a result of reaction with pelitic country-rocks occurred at shallow emplacement level.

               Abundance of ferromagnetic and paramagnetic minerals in granites (sensu lato) determine the magnetic and weakly magnetic properties of felsic igneous rocks, which can be recorded and evaluated in terms of magnetic susceptibility (MS in 10^-3SI unit) and whole rock geochemistry. MS of granites is highly variable and broadly displays bimodal patterns viz. magnetite series (MS >3.0×10^-3SI) and ilmenite series (MS≤3.0×10^-3SI) granites (e.g. Kanaya and Ishihara, 1973; Ishihara, 1977, 1990; Takagi, 2004), corresponding to oxidized- and reduced-type granites respectively (Takagi and Tsukimura, 1997), which are parallel to  metaluminous (I-type) and peraluminous (S-type) granites (Chappell and White, 1974; Takahashi et al., 1980). The dominance of magnetite  and ilmenite  as an indicator of magnetite series and ilmenite series granites respectively is primarily controlled by oxygen fugacity (fO₂) of magma source, which may be partially or wholly modified by later processes such as assimilation and magma mixing etc. during the ascent and emplacement of felsic magma. Tectonism can also erode out the original magnetic properties of the granites. Consequently, correlation between granite series and granite types need not be similar, and hence MS value as indicator of granite origin should be used with great caution (Gregorová et al., 2003). In this paper, an attempt has been made to record MS value of Kyrdem granitoids and enclaves, and further to evaluate the granite series combining whole rock geochemical database.

              Assam-Meghalaya plateau, erstwhile known as Shillong plateau, is a Precambrian geotectonic shield of northeast India. Medlicott (1869) reported the occurrence of several felsic plutons, named after prime villages, which are located along the Tyrsad-Barapani lineament of the Meghalaya plateau (Fig. 1a, b). Mazumder (1976, 1986) described the geology of Meghalaya plateau and considered these felsic plutons (Mylliem, Nongpoh, Kyrdem and South Khasi) as late- to post-tectonic, fracture-controlled diapirs resulting from episodic thermal events caused by mantle upwelling. Ghosh et al., (1991, 1994, 2005) provided whole rock Rb-Sr isotopic ages for these plutons (Kyrdem 479±26 Ma; Nongpoh 550±15 Ma; Mylliem 607±13 Ma; South Khasi 690±19 Ma), which suggest a protracted thermal event of ca 200 Ma (Proterozoic-Early Palaeozoic; ca 700-500 Ma) in the Meghalaya, probably related to Pan African-Caledonian orogeny. These felsic plutons appear younging in age from southwest to northeast and contain abundant microgranular enclaves (Kumar, 1998).

            Early Ordovician (479±26 Ma) felsic magmatism in the Kyrdem region of Meghalaya plateau, herewith referred to as Kyrdem granitoids (KG), intrudes the Shillong Group and Precambrian gneissic complex forming an oval-shaped plutonic body with longer axis almost trending N-S (Fig 1b). Thermal aureole is poorly developed or covered under the alluvium. KG exhibit very coarse grained porpyritic texture with abundant K-feldspar megacrysts (upto 9 cm long) and subordinate amount of amphibole, biotite, plagioclase and quartz. Size of K-feldspar megacrysts increases from margin (Dwarksuid) to the interior (Kyrdem) of the KG pluton. Late felsic pulses as fine grained granite, leucocratic (aplite) and pegmatite veins intrude the KG at several places. Grey and pink varieties of KG can be recognized, but pink colour of KG is the result of post-magmatic fluids, which have not affected the magnetic properties of KG. Modal composition of KG corresponds to quartz monzonite, monzogranite and granodiorite. KG has been geochemically characterized as metaluminous (I-type) to peraluminous (S-type) granitoids (Ghosh et al., 1991). K-Ar ages (521±13 Ma and 457±12 Ma) of biotites from KG are broadly similar to whole-rock Rb-Sr isochron age (479±26 Ma) indicating that the KG pluton has not suffered any post-emplacement tectono-thermal event (Ghosh et al., 1991), and hence redox state of KG should originally correspond to source region and later magmatic processes. Sikdar and Rahman (1998) have studied the structural state of K-feldspar megacrysts from KG, and have concluded that the core of KG pluton has cooled slowly as compared to the margin.

Fig. 2   (a) Amphibolite xenoliths of various shapes and size in porphyritic (K-feldspar megacrysts) Kyrdem granitoids (KG). Note the felsic net-veining of xenolith. Length of MS meter equals 9.7 cm. (b) Gneissic xenolith in KG. Scale is the same as in (a). (c) Elongated K-feldspar megacrysts bearing microgranular enclave (ME) in KG. Alignment of K-feldspar megacrysts in ME correlate with that of host KG. Length of lens’ cap equals 5.5 cm. (d) Randomly oriented K-feldspar megacrysts of various shapes and sizes. Note a small, platy, pelitic xenolith in KG. Scale is the same as in (c). 

            Enclaves hosted in KG can be classified as xenoliths of country rocks (amphibolite and metasedimentary rocks) mostly confined to margin of the KG pluton (Fig 2a, b) and fine to medium grained, mesocratic to melanocratic, mafic-felsic phenocryst-bearing (Fig. 2c) or phenocryst-free microgranular enclaves  commonly ubiquitous in porphyritic KG (Fig. 2d), exposed in and around Sohliya, Mawbsein and Mawblang regions.  The shape of ME is rounded to elliptical on 2D outcrop, and size varies from a few cm to about one meter across commonly having sharp contacts with felsic host KG. Modal composition of ME belongs to quartz monzonite. ME, as hornblende microdiorite, have been described but have been considered as cogenetic early phase of felsic host KG (Ghosh et al., 1991). Field and petrographic evidences strongly suggest that the ME represent mafic to hybrid coeval magma globules, which were mingled and undercooled into a partly crystalline, relatively cooler felsic host KG, as similarly noted elsewhere (Kumar et al., 2004, 2005a, Kumar and Rino, 2006). Magnetic susceptibility parameter of KG and ME has been preferably investigated to evaluate redox series of comingled felsic melt and ME magma globules.

Analytical method

            MS measurements were taken using SM-20 model of MS meter on smooth rock surfaces of fresh outcrops across the Kyrdem pluton, and the obtained MS values were further corrected according to rock surface unevenness using the recommended factors: 1mm-1.07, 2mm-1.15, 3mm-1.23, 4mm-1.32, 5mm-1.41, 6mm-1.51, 7mm-1.61, 8mm-1.72, 9mm-1.84, 10mm-1.96. The measuring range of MS meter is between 0.000 and 999×10^-3SI, and sensitivity of instrument is 1×10^-6SI units.

Results and interpretation

Magnetic susceptibility variation:

            The corrected MS values of Kyrdem granitoids and enclaves measured in and around various localities are given in table-1. At western contact (Dwarksuid) of the Kyrdem pluton, xenoliths of amphibolite (Khasi greenstone), gneiss, schist and phyllite (Members of Tyrsad Formation belonging to Shillong Group) are found hosted in KG. These xenoliths vary in shape and size from a few cm to several meters across. Amphibolite xenoliths mostly occur as isolated large block in KG, which are cross-cut by several leucocratic felsic veins. Amphibolite xenoliths measure MS values in the range of 28.98-68.20×10^-3SI with an average of 43.56×10^-3SI (N=9), which relate to the presence of high amount of amphibole and magnetite in xenolith. On the other hand, average MS values for gneissic (0.28-0.50×10^-3 SI) and pelitic (0.33×10^-3SI) xenoliths have been measured very low. Along with these xenoliths, KG have also shown very low average MS values ranging between 0.22 and 0.31×10^-3SI, which are related to reduced type, ilmenite series granites. The presence of large-size amphibolite xenoliths in KG suggests shallow-level emplacement of KG pluton and probably the stopping of xenoliths from wall and/or roof of the magma chamber. The temperature of KG melt was not high enough to assimilate the amphibolite (metabasalt), whereas gneissic and pelitic xenoliths (as deeper-derived lithology and/or country rocks) have been partially assimilated, and consequently KG melt near the margin of pluton has reduced to ilmenite series granite as similarly observed in the case of South Khasi felsic pluton (Kumar et al., 2005b). However, average MS values of KG away from the contacts in various localities (spot I to XV, table 1) have increased by many folds, mostly varying between 15.56 and 31.55×10^-3SI, which are moderately to highly oxidized, magnetite series granite. It is interesting to note that KG with less abundant K-feldspar, in the Mawlasnai locality (spot X), measures relatively lower average MS values (15.78×10^-3SI, N=4) as compared to KG with abundant K-feldspar megacrysts (21.46-31.55×10^-3SI). This is because K-feldspar megacrysts poikilitically enclose granular magnetite and ferromagnesian minerals in a zoned manner suggesting its magmatic origin and participation in mafic-felsic magma mixing and ME mingling events (e.g. Kumar et al., 2005a, Vernon and Paterson, 2008). Aplite (at Mawblang, spot III) and pegmatite (at Mawbsein, spot IV) veins measure average MS values of 4.53×10^-3SI (N=14) and 5.97×10^-3SI (N=5) respectively both corresponding to low oxidized, magnetite series granites. KG containing variable amount of mafic minerals on a single outcrop (at Mawbsein, spot IV) have shown variable MS values ranging from 18.35 to 47.92×10^-3SI with an average MS of 25.81×10^-3SI (N=14). The maximum recorded MS value (47.92×10^-3SI) of KG contains highest amount of mafic minerals. It is therefore likely that modal abundance of magnetite, biotite, and amphibole in KG control the MS variations.

Table 1: Magnetic susceptibility (MS) measurements of Kyrdem granitoids (KG) and associated rocks

PART-I

PART-II

The MS values given as bold represent range of min-max MS values. N is number of MS measurements.

                Microgranular enclaves (ME) of various shapes and sizes are more frequent in northeastern part of the KG pluton, in and around the Sohliya region. Broadly two types of ME (>20 cm across) with variable MS values of 25.23×10^-3SI (N=6) and 32.09×10^-3SI (N=8) can be recognized, which are related to their mesocratic and melanocratic nature respectively. A relatively small size ME (~10cm across) occuring in Mawbsein area (spot IV) measures MS in the range of 14.65-34.45×10^-3SI with an average MS value of 24.47×10^-3SI (N=10). Majority of MS values for KG and ME represent moderately to strongly oxidized, magnetite series granites. High MS values of ME at least suggest the origin of ME unrelated to xenoliths (amphibolite, gneiss etc.) hosted in KG. However, solely based on MS values a magma-mixed or autolith (cognate) origin for ME in KG cannot be established. The hybridization (active mixing region) commonly prevails in open magma system mostly common in a calc-alkaline, I-type felsic pluton, and it is probable that mafic and felsic magma interaction, as evident by the presence of ME,  might have also played an important role in raising the oxidizing level from moderately to strongly oxidized, magnetite-series of bulk KG pluton.

 Biotite in granite:

              Magnetite series granites may contain biotite (Z colour: greenish brown) rich in Mg because iron is consumed to form early magnetite rich in Fe³⁺ whereas Z colour in biotite of ilmenite series granites is reddish-brown because of its Fe²⁺rich and Fe³⁺poor compositions (Ishihara, 1998). In Kyrdem pluton, mostly KG and ME contain abundant greenish-brown biotites, and are found associated with magnetite, which have high MgO/FeO ratio, and are therefore derived originally from oxidizing source region. In Dwarksuid region near the margin of KG pluton the ilmenite series granite contain high amount of reddish brown biotite (low MgO/FeO) dominating over greenish brown variety, which have been precipitated from a reducing KG magma as a result of country rock assimilation by originally oxidized KG melt.

Whole-rock Fe₂O₃/FeO ratio:

              Whole-rock Fe₂O₃/FeO ratio of 0.5 at 70% SiO₂ of granitoids can be empirically used to distinguish between magnetite series (Fe₂O₃/FeO>0.5) and ilmenite series (Fe₂O₃/FeO<0.5) granites because oxidation state of iron is a primary expression of redox state of magmas commonly demarcated at fayalite-magnetite-quartz buffer (Ishihara, 1977; Ishihara et al., 2000; Ishihara et al., 2002a, b, Hart et al. 2004). Ghosh et al. (1991, Table 2) have presented major oxide data including Fe₂O₃ and FeO contents of sixteen KG samples. Fe2O3/FeO ratio of KG samples varies from 0.09 to 5.25 in the silica range of 63.01-74.15 wt%, with an average Fe₂O₃/FeO of 1.71 and average SiO₂ of 68.53 wt%. Six KG samples have Fe2O3/FeO ratios below 0.50 (0.09 to 0.44) and ten other KG samples have Fe₂O₃/FeO ratio more than 0.5 in the range of 0.65 to 5.25, representing to ilmenite- and magnetite-series granites respectively. This is further corroborated in terms of Fe₂O₃/FeO vs FeO* (Fig. 3) and (FeO+MgO)-(Fe₂O₃+TiO₂)-MgO ternary components (Fig. 4), which equivocally suggest moderately to very strongly oxidized nature of KG. It is noteworthy that a few KG samples (e.g. 8, 3, 10/km, Fig. 2 of Ghosh et al., 1991) are from the contact with country rocks (schist, phyllite and quartzite). It is, therefore, likely that originally oxidized, magnetite series nature of KG melt has locally reduced to ilmenite series granite because of assimilation of the metasediments near western (Dwarksuid) and northern (Raitong-Mawhati) marginal parts of the KG pluton, which are consistent with the observed low MS values and nature of biotite in KG.

Fig. 3:  Redox classification scheme plotted for Kyrdem granitoids (+). FMQ and TMQ-HIL buffer lines and fields of VSO: very strongly oxidized, SO: strongly oxidized, MO: moderately oxidized, MR: moderately reduced, SR: strongly reduced, are taken from Hart et al. (2004).

Fig. 4:   Magnetite series (grey shaded region) and ilmenite series (dotted region) granites of Japan as shown by Shunso Ishihara (pers. comm. 2005), are compared with KG (+) encircled by solid line. Magnetite and region of biotite and amphibole (as shown by broken lines) compositions are taken from Ishihara (1971).  

 Alumina Saturation Index:

         Alumina saturation index (i.e. A/CNK=Molar Al₂O₃/CaO+Na₂O+K₂O) of granitoids has been commonly used to distinguish between metaluminous (I-type, A/CNK<1.05) and peraluminous (S-type, A/CNK>1.05) granitoids (Chappell and White, 1974). About 60% of I-type granitoids of Lachlan Fold Belt, Australia belong to ilmenite series granite, however there should be a parallelism between ilmenite-/magnetite-series and S-/I-type granitoids (Takahasi et al., 1980). In Japan nearly all the ilmenite series granites are metaluminous in nature, and hence magnetite-/ilmenite-series classification does not exactly correspond to the S-/I-type classification scheme (Takahasi et al., 1980; Ishihara, 1998). In Kyrdem pluton, nine KG samples (Table-2, Ghosh et al., 1991) correspond to I-type (metaluminous) granitoids (A/CNK=0.77-1.02), and their Fe₂O₃/FeO ratios mostly comply with magnetite-series granites (Fe₂O₃/FeO >0.5) except for one KG sample. Seven other KG samples are peraluminous (S-type) granitoids  (A/CNK=1.08-1.15), which mostly correspond to ilmenite-series granites (Fe₂O₃/FeO <0.5) except two KG samples with substantially high Fe₂O₃/FeO ratios (5.25 and 2.54). For the KG samples, which do not exactly comply with classification parameters in terms of A/CNK, Fe₂O₃/FeO, MS values etc., it is likely that some factors such as assimilation of carbonaceous pelitic materials at local shallow emplacement level, other than protolith involvement, might have played significant role in the evolution of reduced, ilmenite series KG melt during its shallow emplacement level.

        Kyrdem granitoids (KG) and micrgranular enclaves (ME) are moderately to strongly oxidized, magnetite series granites, which are mostly inherited from their source regions. However, hybrid and mingled nature of ME indicate that oxidation state of KG melt was partly elevated under a coeval interacting mafic-felsic magma system. The bulk of oxidized KG melt was reduced locally near the margin of the KG pluton because of country-rock assimilation at shallow emplacement level.

Acknowledgements: DST-New Delhi grant (ESS/23/VES/046/98) supported this work. Dr. V. Rino, Dr. Brajesh Singh, Mr. N. Surdas Singh and Ms. Manjari Pathak are thanked for extending help in the preparation of the manuscript. Dr. Shunso Ishihara is thanked for providing valuable literature on granite series.

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