A-type Granites from the Nagarparkar Complex, Pakistan: Geochemistry and Origin

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.