Petrology of Neoproterozoic Nagar Parkar Alkaline
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69 Geol. Bull. Punjab Univ. 43, 2008. A-TYPE GRANITES FROM THE NAGARPARKAR COMPLEX, PAKISTAN: GEOCHEMISTRY AND ORIGIN BY SYED ALIM AHMAD Institute of Geology, University of the Punjab, Quaid-i-Azam Campus, Lahore-54590 Pakistan Email: [email protected] AND MOHAMMAD NAWAZ CHAUDHRY Postgraduate Centre for Earth Sciences, University of the Punjab, Quaid-e-Azam Campus, Lahore-54590, Pakistan Abstract: The Neoproterozoic Nagarparkar Complex is characterized by isolated semicircular hillocks of outcrops of “withinplate” A-type peralkaline to peraluminous granites. Geochemically the granites/rhyolites of the Nagarparkar Complex are characterized by high SiO2, Na2O+K2O, K2O, Fe/Mg, Al2O3, Zr, Nb and Y and low CaO and Sr. Majority of samples show moderate iron enrichment. The rocks show the characteristic trend of alkali enrichment and depletion of Ti, Fe, Mg and Ca, which are comparable with the rift-related mildly alkaline suites worldwide. Another characteristic feature of the alkaline/mildly alkaline felsic volcanoplutonics of the Nagarparkar area is their enrichment in incompatible trace elements similar to other peralkaline rocks of the Basin. High values of incompatible trace elements like Zr, Nb, Ce, and Y are characteristic of the granites/rhyolites under study. The multielement primitive mantle normalized spidergrams show that the granites/rhyolite from Nagarparkar Complex are enriched in Rb, Y, U and Zr, a characteristic of A-type granitoids. These granites are also strongly depleted in Ti and Sr. Negative Ti anomaly can be interpreted as reflecting ilmenite fractionation. Nb shows low negative anomaly, which is typical of crustal material. The negative anomaly shown by Ba and Sr can be attributed to low-pressure felspar fractionation and are typical of A-type granitoids. Primitive mantle normalized spider diagrams show that the Nagarparkar granites/rhyolites are enriched in Rb, La, Ce, Zr and Y and depleted in Sr, Ba, Ti, P and show negative Nb anomaly, which is typical of upper crust (hypersolvus). The Chondrite normalized spidergrams show that LREE are enriched as compared with HREE. A wide variation in Eu/Eu*values suggest felspar fractionation. P.I Index of majority of samples varies from .87 to 1. A/CNK versus ANK plots clearly confirm peralkaline to peraluminous nature of the rocks. The presence of Na-amphibole in the granites further confirms the alkaline nature of the granites. Granites/rhyolite plot as “alkali granite” in the QAP diagram. The normative appearance of acmite and nepheline in some samples further supports the alkaline nature of these rocks. The granites/rhyolites of the Nagarparkar Complex exhibit “within plate” setting on plotting in different discrimination diagrams. INTRODUCTION Eby, 1990; Rathore et al., 1991; Butt et al., 1992 and Ahmad et al., 2000). This study consists of geochemistry of The peralkaline rock complexes of the West Indian the peralkaline granites of the Nagarparkar Complex Shield at Thar and Rajasthan areas on the both sides of the constitutes a distinct anorogenic igneous cratonic rift Indo-Pakistan border, has isolated hillocks of magmatism assemblage of the widespread magmatic activity and is a represented by the Nagarparkar, Malani, Kirana, Siwana, part of widespread Late Proterozoic Malani Igneous suite Tosham, Mount Abu and Gurapratapsingh, complexes extending from western Rajasthan (India) to Sindh Province (Kochhar, 1973, 1974, 1984; Bushan.1985, Naqvi. 1987; in Pakistan. The Complex constitutes two distinct magma 70 places, particularly around Dhedvero, they are highly series; the older series represents the plutonic subalkaline weathered, sheared and show foliation. Present field association of gabbro, diorite, granodiorite and granites, investigations and petrographic studies clearly indicate that while the younger volcanoplutonic magma series represents the gabbroic intrusions locally have been subjected to pulses of bimodal suites comprising alkali dolerite and variable alteration and weathering. Near Dhedvero, alkali rhyolite/granite. These intrusions of tholeiitic and chloritization and epidotization is generally observed. subalkalic mafic dykes and coeval magmas in the Nagarparkar Complex are related to the Neoproterozoic Typical adamellites are exposed in Wadlai area. Jan rifting of NE Gondwanaland, and are therefore, et al (1997) coined the term ‘Mottled’ pink granite for this fundamental in understanding the interaction between adamellite rock found NE of Mokrio village. Petrographic magma genesis and the geodynamics of sub continental and geochemical studies shows that the rock is a medium to lithospheric mantle beneath the Northwestern margin of the coarse-grained alkali adamellites rather than adamellite as Indo-Pakistani plate. believed earlier (Muslim et. al, 1997). The Nagarparkar Complex is a multi-intrusion The Nagarparkar Granite occurs near Nagarparkar mildly alkaline magma association of gabbro, alkali-granite, town along the northeastern margin of Karunghar Hill. emplaced probably as a horseshoe shaped ring dyke Other exposures of this unit are found in Bhodisar, Dinsy, complex. Such complexes are exclusively confined to the Waravai Dhedvero and NE of Kharsar. The granite is rift-valley system of extensional environment worldwide medium to coarse grained, mostly homogenous and (Le Bas, 1986 and Woolly, 1992). It is generally believed leucocratic. It exhibits spheroidal weathering surfaces. The that such alkaline complexes are formed as a result of granite in the Nagarparkar and Bhodisar is intruded by continental rifting, characterized by alkaline differentiates. aplites, acid porphyry dykes and doleritic dykes (Kazmi and Khan, 1973; Butt et al., 1992 and Muslim et al., 1997). This paper presents the geochemistry of the granites Several small dykes of felsic composition are found alongwith crystallization history from the Nagarparkar intruding the older lithologies, particularly in Dhedvero and Complex based on major and trace element geochemistry Nagarparkar localities. They range from rhyolite porphyry and microprobe analyses. to aplite and microgranite. Best exposures of acid porphyry GEOLOGICAL SETTING and microgranite dykes occur in Dhedvero, Chanida, Dhanagam Ghantiari and Dinsy areas. The Nagarparkar Complex is situated at the extreme Major outcrops of rhyolite and microdiorite occur southeast of Thar Desert of Sindh near the Runn of Kutch near Sadarous and Dungri-in the form of domal shape (24º 15' to 24º 30' latitude, 70º 37' to 71º 07' longitude) and plugs. Small bodies of rhyolite unit are also found covers an area of about 450 sq km (Fig.1). The earliest associated with the Grey Granite and the granitic stocks in accounts of the geology of the Nagarparkar rocks are Dinsy and near Mokrio. The rhyolite unit is homogeneous, recorded in Geological Survey of India (Wynne, 1867 and dark grey and dark brown in colour, fine-grained, Fermor, 1932). A map of the area and description of the subporphyritic at places glassy-looking and apparently basic intrusions and granitic rocks were presented by Kazmi banded. and Khan (1973) who defined the Nagarparkar Complex as Precambrian basement. Description of geology and Grey Granite covers the major portion of Karunghar petrographic data of various rock units of the Nagarparkar Hill, forming a large plutonic body striking in the NW-SE Complex are presented by several workers (Shah, 1973; direction. The exposures of this unit are found at many Butt et al., 1989, 1992; Muslim et al., 1997 and Jan et al., places including Karai, Dungri, Ghantiari, Rarko, Karai, 1997). These workers are of the opinion that the Adhigam and Mau. The Grey Granite is undeformed; Nagarparkar Complex is a part of Precambrian fragment of medium to coarse grained, contains xenoliths of dolerite the Western Indian Shield. The granites of the complex are and is characterized by peralkaline mineralogy (Na- considered to be the extension of the post-Aravalli amphiboles are present in most of the samples studied) magmatism in the Late Proterozoic. They may have been Churio Granite occurs as isolated outcrops in Churio generated as a consequence of sinistral shear along the and Virawah. It is coarse grained, off-white to light pink in Narmada-Son lineament and crustal anataxis in the colour, leucocratic and shows alkaline character. This type continental anorogenic environment (Butt et al., 1997 and of granite is also exposed at Mokrio, Sadarous and Dungri. Jan et al., 1997). The basement may have undergone a GEOCHEMISTRY tectono-metamorphic event before the emplacement of the granites. The major and trace element analyses are given in Table. 1. The chemistry of the Nagarparkar Complex is The gabbroic intrusions occur near Dhedvero, Karai, Karki, Parodhra and Ramji-Ka-Vandi areas. They are dark green massive and generally coarse to medium-grained. At 71 72 Table 1. Major and trace elements analyses of granite from Nagarparkar Complex Grey Granite (Nagarparkar Complex) Sample# S-875 M-913 N-946 N-948 N-952 N-953 N-1954 N955 N-956 MAJOR ELEMENTS SiO2 77.64 76.59 76.83 78.53 78.50 78.12 75.46 76.96 78.18 TiO2 0.13 0.24 0.22 0.14 0.13 0.11 0.13 0.13 0.14 Al2O3 11.93 11.56 11.84 11.29 11.22 11.69 11.74 11.36 11.53 Fe2O3(t) 1.19 2.75 2.51 2.16 2.34 1.96 1.68 2.09 1.80 FeO(t) 1.07 2.48 2.26 1.94 2.11 1.76 1.51 1.88 1.62 Fe2O3 0.54 1.24 1.13 0.98 1.06 0.89 0.76 0.94 0.81 FeO 0.59 1.37 1.25 1.08 1.16 0.98 0.84 1.04 0.89 MnO 0.03 0.07 0.05 0.02 0.05 0.03 0.04 0.03 0.03 MgO 0.14 0.11 0.19 0.10 0.12 0.11 0.17 0.13 0.12 CaO 0.39 0.28 0.48 0.11 0.23 0.14 0.45 0.14 0.26 Na2O 3.92 3.83 3.46 3.18 3.31 3.73 6.31 5.09 3.49 K2O 4.69 4.71 4.54 4.58 4.22 4.30 4.05 4.17 4.54 P2O5 0.01 0.02 0.01 0.01 0.02 0.01 0.05 0.02 0.01 Mg.