Geology and Geochemistry of Neoproterozoic Kirana Volcanics, Sargodha District, Pakistan
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49 Geol. Bull. Punjab Univ. 43, 2008. KIRANA VOLCANICS, PAKISTAN-GEOCHEMICAL CHARACTERIZATION AND ORIGIN BY SYED ALIM AHMAD Institute of Geology, University of the Punjab, Quaid-i-Azam Campus, Lahore-54590 Pakistan Email: [email protected] AND MUHAMMAD NAWAZ CHAUDHRY Postgraduate Centre for Earth Sciences, University of the Punjab, Quaid-a-Azam Campus, Lahore, Pakistan. Abstract: Kirana Volcanics are a part of the isolated hillocks of the post-Aravalli sequence, called “MIP” (Malani Igneous Province) in India and “Kirana-Malani Basin” in Pakistan.50 samples of rhyolitic lavas have been analysed geochemically from the Neoproterozoic (870±40) Kirana Complex, district Sargodha, Pakistan. These rocks are intercalated with their mafic counterparts. Geochemically the rhyolites have high-K and are sub-alkaline in character. “Within plate” and “A-type affinity has been proposed for these rocks on the basis of geochemistry. A general LFS vs HFS element fractionation alongwith a positive Ta anomaly similar to A- type granites is clearly observed. As a result of existence of compositional gap between the silicic and mafic magmas in the MIP and KMB the formation of silicic lavas is attributed to anataxis of an intermediate crust. The REE fractionation implies a marginally lower degree of melting for the lava forming magma. On the basis of geochemical data the rhyolitic rocks have been divided in to two groups, each derived from a different protolith from within an intermediate crust. Eu correlates negatively with the crystal abundance of each rhyolitic facies. three phases of igneous activity have been identified in the INTRODUCTION Complex (Ahmad et al., 2004). The first commenced with Kirana Volcanics are a part of the Neoproterozoic the eruption of basic flows, followed by voluminous acid Kirana Complex (Ahmad et al., 2000, 2004). The outcrops flows, which culminated with ash flow deposits. The of this complex occur as isolated hillocks around Sargodha, second phase is represented by intrusion of peraluminous covering toposheet no 44A/9 and 41A/3 and lie between and peralkaline granites as plutons and ring dykes while the longitudes 72°-38´-48" to 72°-48´-00" and latitudes 31°-51´ third phase is characterized by intrusion of mafic and felsic 00" to 32°-15´-00" (Fig.1). The whole complex of hillocks dyke swarms. The bulk of the volcano-plutonic rocks are now represented by isolated groups on the Post-Aravalli overlain by a well-developed sedimentary sequence in sequence has been designated as Kirana-Malani Basin Kirana area (Ahmad et al., 2004). (Ahmad et al., 2000). These isolated hillocks (called Malani The present study focuses on a Neoproterozoic Igneous Suites in Indian literature) occur as hillocks and scattered hummocks covering areal extent of around silicic volcanics with dolerites of the Kirana Complex. The 162000 sq. km (including area below cover sediments) in rhyolitic component comprises rhyolitic tuffs, rhyolite Rajasthan, Thar and Punjab Plain covering parts of porphyries, welded tuffs and ignimbritic sheets associated Jaisalmer, Barmer, Jodhpur, Kirana, Pali, Jalore, Bikaner, with silicic lava extrusions, which erupted simultaneously Sirohi and Nagar areas. The whole region is composed of with basaltic lavas. Rocks of andesite composition, though volcanoplutonic association of granite and rhyolite as major present are rare. This paper is based on the results of field, rock units along with gabbros and dolerites. Based on field petrographic and geochemical studies on the rhyolites of the relationships, mode of occurrence, texture and composition, Kirana Complex. 50 51 PETROGRAPHY 128,and 120. Here we find microphenocrysts of quartz and K-felspar are surrounded by very fine anhedral epidote and The volcanics are fine grained and occasionally microcrystalline groundmass composed of quartz and massive. The rhyolites are characterized by phenocrysts of microcrystalline matter. Generally microcrystalline calcite quartz and feldspar and rarely ferromagnesian mineral occurs in the groundmass as well as in crystals, filling phases in microcrystalline or glassy matrix. The rhyolites cracks and other voids. Plagioclase is subhedral to euhedral are generally merrocrystalline, microporphyritic and fluidal. and occurs as phenocrysts as well as small crystals in the Micro vesicular varieties are also common. The rhyolites, groundmass. It is variably altered to epidote and sericite. rhyolitic tuffs and rhyolite porphyries are major Plagioclase laths may enclose chlorite and anhedral epidote. representatives of silicic volcanic rocks. Generally these Sometimes laths of plagioclase are embedded in rhyolites are merrocrystalline to microcrystalline and rarely microcrystalline/fine quartz groundmass forming holo-hyaline. Notwithstanding physical appearance the microphenocrystic texture. At places tiny plagioclase laths volcanics are predominantly rhyolitic in petrographic along with quartz are enclosed by K-felspar perthite composition. Generally the rocks are microgranular and phenocrysts. Microphenocrystic texture is common in composed of minute granules of quartz, altered K-felspar, samples of hills of Chak 123, 128,and 120. Quartz occurs as chlorite and zoisite. Sometimes secondary calcite is present strained, anhedral grains and microphenocrysts. It may as fracture filling and in the groundmass. enclose calcite and contain gas and fluids inclusions. Ilmenite/leucoxene occurs as subhedral fine crystals. Epidote is fine grained and from anhedral to subhedral. Occasionally microliths of altered felspar are also seen. Both zoisite and clinozoisite are present. There are Sometimes proportion of microliths increases and the secondary minerals formed due to the alteration of matrix to these microliths is microgranular. The plagioclase. reconstituted texture of the matrix suggests that it represents devitrified and recrystallised glass. GEOCHEMISTRY Tuffs are fairly prominent and easy to identify in the 50 rhyolite samples from representative textural field. Generally two textural varieties are identified i.e. varieties were systematically collected across the eruptive lithic felsitic and vitric felsitic. Generally these rocks are sequence of the Kirana Complex. Analyses of the major and typically merrocrystalline i.e. with glassy/spherulitic trace elements were carried out by wet chemical, AAS, groundmass and fluidal structure in which are enclosed XRF and INAA in the laboratories of the Pakistan Institute fragments of K-feldspar/plagioclase and quartz. of Engineering and Applied Sciences Islamabad and Hematite/goethite grains may occur along flow foliation. GeoScience laboratories Islamabad. The analyses of the The vitric variety of tuff includes compacted tuff, in which representative samples are given in the Table. 1. shardy fragments have not been deformed, and also welded tuffs or ignimbrites in which considerable plastic flattening The chemical classification of the Kirana Complex of the original shardy elements has taken place. The is based on the total alkali-silica diagram (TAS-Fig. 2) of presence of later suggests that the deposition of the pile LeBas et al. (1986), LeMaitre (1989) and Middlemost took place under sub-areal conditions. Mineralogical (1991). The SiO2 boundaries were defined using an composition of tuffs suggests that they have been derived equivalent diagram of Ewart (1982). SiO2 versus Zr/TiO2 from magma of acidic composition. and Nb/Y versus Zr/TiO2 diagrams (Figs-3, 4 after Winchester and Floyd, 1977) were used as alternate Lava flows are interlayered with slates (often diagrams to check alteration. The igneous rocks of the volcanogenic). Lava is of acidic composition and represents Kirana Complex exhibit subalkaline trend and range in typical potassic rhyolites. Lava flows are wide spread in composition from tholeiite basalt, basaltic andesite, Hachi, Buland, Tuguwali, Shaheenabad, Chak 112, and andesite, and dacite to rhyolite. However the intermediate Chak 128 hills. These are generally greyish, pale green and rocks are a minor component and the complex falls into two light brown in colour having quartz and K-feldspar distinct sets of rocks i.e. mafic suite (dolerite) and felsic microphenocrysts in glassy/microcrystalline groundmass. In rocks (dacite and rhyolite). These rocks represent flows the glass to microphenocryst ratio ranges up to 85:15. subalkaline tholeiitic magma activity. The mafic rock suites K-feldspar occurs as subhedral to eumorphic crystals, mostly straddle the low-K (Fig, 5 after Rickwood, 1989) forming phenocrysts as well as smaller crystals in the and medium-K boundary; with a few samples showing a groundmass, which are variably sericitized. Some trend towards high-K/shoshonites (Fig, 4, 5). With the plagioclase laths enclosing chlorite are embedded in exception of a few random samples of andesitic microcrystalline/fine grained quartz groundmass. In some composition, a distinct compositional gap exists from 56- cases porphyroblasts of K-felspar (perthite) enclose wt% SiO to 66 wt% SiO . Thus the volcanics show anhedral quartz and fine laths of albite. Microperthitic 2 2 bimodality of dolerite-rhyolite association. texture was also observed in some samples from Chak 123, 52 Table. 1. Major and trace elements analyses of the rhyolites from the Kirana Complex TW-613 TW-614 BH-625 FE-630 FE-631 FE-651 FN-669 PD-671 PD-672 T-673 TE-722 SH-731 SiO2 74.15 75.15 77.34 77.18 76.98 76.58 78.46 72.21 79.54 73.67 77.72 77.78 TiO2 0.30 0.50 0.14 0.15 0.16 0.14 0.21 0.26 0.18 0.15 0.65 0.26