Geochemistry and petrogenesis of the Miocene alkaline and sub-alkaline volcanic rocks from the Chagai arc, Baluchistan, Pakistan: Implications for porphyry Cu-Mo-Au deposits

Abstract

The Miocene volcanic rocks are developed in the western part of an EW trending subduction related magmatic belt known as the Chagai arc in the Baluchistan province of Pakistan. The volcanism in this arc initiated during the Late Cretaceous, which intermittently continued up to the Quaternary period. In the regional geotectonic context this arc belongs to the Tethyan convergence zone and is believed to have formed due to the northward subduction of Arabian oceanic plate beneath the southern margin of Afghan micro plate and hence is considered as an Andeantype arc.

The Miocene volcanic and volcaniclastic rocks form a gently dipping caldera of a collapse stratovolcano. This volcanic caldera is intruded by several tonalite porphyry stocks, hosting porphyry copper deposits.  These volcanics are described as Buze Mashi Koh Volcanic Group and are mainly represented by interstratified andesitic lava flows and volcaniclastics including agglomerates, tuffs, lapilli tuffs and volcanic breccia. Towards the northern side of the volcanic caldera, basaltic lava flows are also found.

The petrological studies reveal that Miocene volcanic rocks comprise mainly basaltic-andesite, andesite, basaltic-trachyandesite and trachyandesite. Two distinct series, sub-alkaline (0.73-1.66 wt. % K₂O) and alkaline (2.94-4.69 wt. % K₂O) are recognized based on petrochemical studies. The sub-alkaline basaltic rocks have high magnesium MgO (8.12-11.38 wt. %) Mg # (68-69), Co (28-42 ppm), Ni (286-307 ppm) and Cr (662-720 ppm) suggestive of their direct derivation from the partial melting of a sub-arc mantle source. The alkaline rocks have lower Mg # (44-59) and lower contents of Mg, Co, Ni and Cr. Their primordial mantle-normalized trace element patterns show enrichment in all the incompatible trace elements, marked negative Nb anomalies and generally positive spikes on Ba and Sr, which strongly confirm their island arc signatures. The chondrite-normalized REE patterns show high and variable enrichment of LREE and negative Eu anomalies, suggesting fractionation of plagioclase during differentiation. Plots in various tectonomagmatic discrimination diagrams depict that these rocks were erupted in a continental-margin type arc environment. Enrichment of most incompatible trace element and greatly enhanced Zr/Y (9.67), Ti/V (31.83), Ti/Zr (34.67), Ba/Y (53.29), Th/Y (0.39) La/Yb_N (23.44), Ta/Yb (0.58), Th/Yb (6.01) ratios relative to N-MORB indicate that parent magma of both the volcanic series was derived from an enriched sub-arc mantle source. The Zr versus Zr/Y and Cr versus Y plots suggest that parent magma of these rock suites was generated by the partial melting of about 15-25 % enriched sub-arc mantle source. A comparison of the Oligocene-Miocene volcanic rocks with other analogous rocks of the Chagai arc show relatively enhanced values of LILE and LREE. This suggests that during the Miocene, which is also the time of emplacement of several porphyry copper deposits in the Chagai arc, fluids enriched in LILE and LREE emanating from the subducting slab of oceanic crust were added to the sub-arc mantle source in greater abundances. This high rate of addition of these fluids is postulated to be related with the rapid and shallow subduction of Arabian oceanic plate below the Zagros-Makran convergence zone that might be related with the opening and rapid spreading of the Red Sea during this period. This suggests that during the Oligocene period which is the time of emplacement of several porphyry copper deposits in the Chagai arc; comparatively more LILE and LREE enriched fluids from the subducting slab of oceanic crust were added to the sub-arc mantle source.