TY - JOUR
T1 - Petrogenesis of plagiogranites in the Muslim Bagh Ophiolite, Pakistan
T2 - implications for the generation of Archean continental crust
AU - Cox, Daniel
AU - Kerr, Andrew
AU - Hastie, Alan
AU - Kakarc, Ishaq
PY - 2018/4/2
Y1 - 2018/4/2
N2 - 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 crystallisation 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, north-western 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 Archean TTG (trondhjemite – tonalite – granodiorite) has implications for the generation of juvenile Archean continental crust. As the Muslim Bagh Ophiolite was derived in a supra-subduction zone, it is suggested that some Archean 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, of Archean TTG.
AB - 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 crystallisation 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, north-western 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 Archean TTG (trondhjemite – tonalite – granodiorite) has implications for the generation of juvenile Archean continental crust. As the Muslim Bagh Ophiolite was derived in a supra-subduction zone, it is suggested that some Archean 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, of Archean TTG.
KW - Pakistan
KW - Muslim Bagh
KW - Ophiolite
KW - oceanic plagiogranite
KW - partial melting
UR - https://doi.org/10.1017/S0016756818000250
UR - http://orca.cf.ac.uk/110822/
U2 - 10.1017/S0016756818000250
DO - 10.1017/S0016756818000250
M3 - Article
SN - 0016-7568
VL - 156
SP - 874
EP - 888
JO - Geological Magazine
JF - Geological Magazine
IS - 5
ER -