Petrogenesis of Middle Triassic volcaniclastic rocks from Balochistan, Pakistan: Implications for the break-up of Gondwanaland

Abstract

Basaltic volcanic conglomerates near the Wulgai village in Balochistan occur in the undivided sedimentary rock unit of the Bagh complex which is the mélange zone beneath the Muslim Bagh ophiolite. The presence of Middle Triassic grey radiolarian chert within the upper and lower horizon of the conglomerates suggests that the lavas, from which these conglomerates were principally derived, were eroded and re-deposited in the Middle Triassic. The Wulgai conglomerate contains several textural and mineralogical varieties of volcanic rocks, such as porphyritic, glomerophyric, intersertal and vitrophyric basalts. The main minerals identified in these samples are augite, olivine, plagioclase (An_35-78) leucite and nosean, with apatite ilmenite, magnetite and hematite occurring as accessory minerals. These rocks are mildly to strongly-alkaline with low Mg^# and low Cr, Ni and Co contents suggesting that their parent magma had undergone considerable fractionation prior to eruption. Trace element-enriched mantle-normalized patterns with marked positive Nb anomalies are consistent with 10%-15% melting of an enriched mantle source in a within-plate tectonic setting. It is proposed that this Middle Triassic intra-plate volcanism may represent mantle plume-derived melts related to the Late Triassic rifting of micro-continental blocks (including Afghan, Iran, Karakorum and Lhasa) from the northern margin of Gondwana.

Keywords

Middle Triassic / Wulgai volcaniclastics / juvenile Ceno-Tethys

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