Lower Indus BasinNagar Parkar igneous complex:
Jan et al. (1997) named and described a sequence of acid and basic igneous rocks exposed near Nagar Parkar in Sind as “Nagar Parkar igneous complex”. Earlier Kazmi and Khan (1973) had called it as “Nagar Igneous complex”, “Late Proterozoic rocks of Nagar Parkar” by Butt et al. (1994), “Nagar Parkar granite” by Shah (1977), and “Nagar Parkar Massif’ by Muslim and Akhtar (1995). None of these names have yet been formalized by Stratigraphic Committee of Pakistan. Area at a glance shows granite of grey and pink colours,intruded into basic igneous rocks,which are in turn intruded by mafic dykes.
Butt et al. (1994) mapped and described the rock sequence as follows: “The oldest rocks are
metamorphosed basic igneous rocks. Acid dykes have intruded these metabasites, which have the level of epidote amphibolites metamorphism. Acid dykes range from rhyolite to quartz trachyte in composition. Metabasites are intruded by grey granite, which contain the xenoliths of the former rock type. Pink granite intrudes the grey granite, and also carries xenoliths of mafic rocks, finally all the major rocks including both the granites are intruded by mafic dykes”. Jan et al. (1997) made detailed and critical petrographic studies and produced a geological map of the area. According to them six major magmatic episodes of intrusive and extrusive activities have been identified.
Jan et al. (1997) named and described a sequence of acid and basic igneous rocks exposed near Nagar Parkar in Sind as “Nagar Parkar igneous complex”. Earlier Kazmi and Khan (1973) had called it as “Nagar Igneous complex”, “Late Proterozoic rocks of Nagar Parkar” by Butt et al. (1994), “Nagar Parkar granite” by Shah (1977), and “Nagar Parkar Massif’ by Muslim and Akhtar (1995). None of these names have yet been formalized by Stratigraphic Committee of Pakistan. Area at a glance shows granite of grey and pink colours,intruded into basic igneous rocks,which are in turn intruded by mafic dykes.
Butt et al. (1994) mapped and described the rock sequence as follows: “The oldest rocks are
metamorphosed basic igneous rocks. Acid dykes have intruded these metabasites, which have the level of epidote amphibolites metamorphism. Acid dykes range from rhyolite to quartz trachyte in composition. Metabasites are intruded by grey granite, which contain the xenoliths of the former rock type. Pink granite intrudes the grey granite, and also carries xenoliths of mafic rocks, finally all the major rocks including both the granites are intruded by mafic dykes”. Jan et al. (1997) made detailed and critical petrographic studies and produced a geological map of the area. According to them six major magmatic episodes of intrusive and extrusive activities have been identified.
(1) Amphibolites and related dykes: the amphibolites, apparently forming host rock for the subsequent intrusions, show low-grade metamorphism. Geophysical survey carried out by Farah and Jaffrey (1966) in the area indicates that they cover large subsurface area and evidently form the basement for the later rocks; they are metamorphosed up to amphibolite grade.
(2) Riebeckite-aegirine grey granite: The riebeckite-aegirine grey granite is essentially composed of perthitic feldspar and quartz, with a small amount of plagioclasc and presence of sodic minerals including characteristic riebeckite and aegirine. The best exposures of the grey granite are at Karunjhar Hill. It is mostly undeformed;also contain iron oxide, zircon and many other accessory minerals. Jan et al. (1997) correlated this unit with Warsak and Sheva-Shahbaz Garhi granite in the northern Pakistan.
(3) Biotite-homblende pink granite: The biotite-homblende pink granite is mostly medium
to coarse-grained. Jan ct al. (1997) termed it as “generally leucocratic and commonly homogeneous”. It is generally made up of “light minerals” like perthitic feldspar, local microcline, quartz, and minor plagioclase (oligoclase), with some biotite, hornblende and iron oxide. Some rocks contain sufficient plagioclase to be termed quartz monzonite or adamellite.
(4) Acid dykes: This unit includes porphyritic microgranite dykes, rhyolitic dykes and rhyolite porphyry and with leucogranitic and equigranitic aplitedykes. In brief,this unit ranges from aplite to microgranite to quartz trachyte. It contain phcnocrysts of perthite, plagioclase and quartz in an allotriomorphic matrix of these minerals and it induces in it accessory minerals, like iron oxide, blue-green amphibole (riebeckite), biotite, zircon, apatite, fluorite, sphene, allanite and secondary epidote. These rocks generally occur as small bodies but locally (as in Dhedhvero), form up to 6 m thick dykes extending for more than 2 km.
(5) Rhyolite Plugs: The rhyolites occur in two small, domel outcrops surrounded by alluvium. They arc dark grey to black, glassy looking rocks with whitish bands and consist of phenocrysts of feldspar and quartz in a very fine-grained matrix. The rocks are fine-grained and porphyritic to sub porphyritic, contain phenocrysts of K-feldspar, mainly perthite and quartz with small amount of plagioclase and accessory minerals like zircon and apatite.
(6) Basic dykes: All the major rock units of the complex are intruded by undeformed basic dykes, mostly less than 3 m thick. They show considerable petrographic variation and range from hornblende microdiorite to gabbro and dolerite, some of which contain titanian augite suggestive of alkaline affinity(Janetal., 1997).
Broadly, two types of dykes have been encountered in the area: lamprophyric and alkaline gabbro/dolcrite dykes. These fnafic dykes are the youngest cutting across all the exposed rock units
in the area. The Nagar Parkar granite is believed to be the continuation of the Proterozoic granitoids
of the Indian Rajasthan.
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