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Petrogenesis of plagiogranites in the Muslim Bagh Ophiolite, Pakistan: implications for the generation of Archaean continental crust

Published online by Cambridge University Press:  02 April 2018

DANIEL COX Open the ORCID record for DANIEL COX [Opens in a new window] ,
ANDREW C. KERR ,
ALAN R. HASTIE  and
M. ISHAQ KAKAR
DANIEL COX*
Affiliation:
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK
ANDREW C. KERR
Affiliation:
School of Earth and Ocean Sciences, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK
ALAN R. HASTIE
Affiliation:
School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
M. ISHAQ KAKAR
Affiliation:
Centre of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan
*
†Author for correspondence: [email protected]
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Abstract

High-SiO2 rocks referred to as oceanic plagiogranites are common within the crustal sequences of ophiolites; however, their mode of petrogenesis is controversial with both late-stage fractional crystallization and partial melting models being proposed. Here, we present new whole-rock data from plagiogranitic dyke-like bodies and lenses from the lower and middle sections of the sheeted dyke complex of the Cretaceous Muslim Bagh Ophiolite, northwestern Pakistan. The plagiogranites have similar geochemical signatures that are inconsistent with them being the fractionation products of the mafic units of the Muslim Bagh Ophiolite. However, the plagiogranites all display very low TiO2 contents (<0.4 wt%), implying that they formed by partial melting of mafic rocks. Melt modelling of a crustal gabbro from the Muslim Bagh Ophiolite shows that the trace-element signature of the plagiogranites can be replicated by 5–10% melting of a crustal hornblende gabbro with amphibole as a residual phase, resulting in a concave-up middle rare Earth element pattern. Compositional similarities between the Muslim Bagh Ophiolite plagiogranites and Archaean TTG (trondhjemite–tonalite–granodiorite) has implications for the generation of juvenile Archaean continental crust. As the Muslim Bagh Ophiolite was derived in a supra-subduction zone, it is suggested that some Archaean TTG may have been derived from melting of mafic upper crust in early subduction-like settings. However, due to the small volume of Muslim Bagh Ophiolite plagiogranites, it is inferred that they can be instructive on the petrogenesis of some, but not all, Archaean TTG.

Keywords

PakistanMuslim Baghophioliteoceanic plagiogranitepartial melting
Type
Original Article
Information
Geological Magazine , Volume 156 , Issue 5 , May 2019 , pp. 874 - 888
Copyright
Copyright © Cambridge University Press 2018 

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