Petrology and geochemistry of amphibolites and greenschists from the metamorphic sole of the Muslim Bagh ophiolite (Pakistan): implications for protolith and ophiolite emplacement

Abstract

Metamorphic sole rocks are exposed beneath both the Jang Tor Ghar Massif (JTGM) and Saplai Tor Ghar Massif (STGM) of the Muslim Bagh ophiolite. The sole rocks comprise the basal mylonitic part of the ophiolite peridotites and the sub-ophiolitic metamorphic rock series showing inverted metamorphic gradients. The latter mainly consist of garnetiferous amphibolites, amphibolites and greenschists. The mineralogy of the amphibolites (hornblende + plagioclase ± quartz ± biotite ± epidote ± apatite ± opaque) and garnet amphibolites in the metamorphic sole rocks of the Muslim Bagh ophiolite is similar except for the presence of garnet in the latter. Greenschists contain minerals such as chlorite + plagioclase + epidote ± actinolite ± quartz ± opaques. The mineral assemblages of these rocks suggest that they are meta-basites. Geochemical analyses indicate that the garnetiferous amphibolites are metamorphosed tholeiitic to alkaline basalts, akin to ocean island basalts (OIB). By contrast, the amphibolites and greenschists have geochemical signatures akin to mid-oceanic ridge basalts (MORB). Basalts of OIB type are also found in the hyaloclastite-mudstone unit (Bhm), while the MORB-type basalts are found in the basalt-chert unit (Bbc) of Bagh complex underlying the ophiolite nappe. Here, we interpret an early stage OIB-type basalt accretion to the base of the obducted plate associated with extrusion of volcanic rocks in the Bhm unit of Bagh complex followed by amphibolite facies metamorphism. During the later stage of the advancing ophiolitic thrust sheet, MORB-like basalts, such as those found in the Bbc unit of the Bagh complex, are underplated and metarmophosed to greenschist facies with subsequent accretion of the entire sequence of the Muslim Bagh ophiolite and the Bagh complex onto the Indian Platform sediments.

Appendix

Analytical methods

Thirty samples (ten samples each from garnet amphibolites, amphibolites and greenschists) were studied under microscope. For both the petrography and geochemistry, the samples were collected from the well-foliated garnet-free and garnet-bearing amphibolites and chlorite schists assuming to be metabasites. The samples were prepared in the rock cutting laboratory of the Centre of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan where they were chipped to the size of about 4 × 2 cm using Metkon Geocut geological cutter. Where needed once again the chips were cut to minimise their thickness. Using silicon carbide, the chips were ground and polished with the Metkon forcipol 300-1-V grinder and polisher to the standard 0.03 mm size. The thin sections were covered with cover slips. The microscopic studies were carried out in the petrography laboratory, Centre of Excellence in Mineralogy, University of Balochistan, Quetta, using Olympus BX51 optical transmitting light microscope.

Geochemical methods

Twenty samples (six from garnet amphibolites, five from amphibolites and nine from greenschists) were analysed for major and trace elements and thirteen samples (five from garnet amphibolites, two from amphibolites and six from greenschists) were analysed for REE. After removing the weathered surfaces, the samples were crushed in a jaw crusher; they were powdered in a tungsten carbide mill to <0.074 mm. The required number of grams of the powder of each sample was heated in a porcelain crucible to 900 °C for 2 h to determine the loss on ignition. For major elements, the sample powder was thoroughly mixed with lithium tetra-borate (flux) with a 1:5 sample flux ratio and the glass beads were formed. For trace elements, powdered pellets of all samples were prepared by taking 5–7 g of powered sample (<0.074 mm) using a hydraulic press. Both the fused beads and pressed pellets were analysed using a Philips Wave length Dispersive X-Ray Fluorescence (WD/XRF) at the Geoscience Research Laboratories, Geological Survey of Pakistan, Islamabad. For REE study, the powder was dissolved in 30 mL of 10 % HNO₃ and 20 mL of de-ionised water. The samples were prepared by a standard teflon vial acid digestion method using a mixture of HF±HClO₄–HNO₃. All samples were spiked using 50 ng/mL of Indium to serve as an internal standard. REE analyses were calibrated against a set of multi-element working standard solutions. All the solutions were introduced via a peristaltic pump, and analyses were performed by using a Perkin-Elmer ELAN 6100 Inductively Coupled Plasma Mass Spectrometer (ICP-MS) at the Amdel Laboratory, Pvt. ltd, Adelaide Australia. The results from the geochemical analyses are reported in Table 1.