Geochemistry of Permian–Triassic shales in the Salt Range, Pakistan: implications for provenance and tectonism at the Gondwana margin

Abstract

Upper Permian and Lower Triassic (Griesbachian, Dienerian, Smithian and Spathian) shales in the Salt Range, Pakistan, were deposited at the margin of Gondwana during a time of active rifting. XRF and INAA major element, trace element and rare earth element (REE) analyses of shales exhibit progressive change in chemistry from Upper Permian to Spathian. SiO 2/Al 2O 3 decreases upsection, and K 2O/Na 2O increases, together with an increase in the Chemical Index of Alteration (CIA). HFS elements are enriched relative to PAAS at the base of the sequence, but are progressively depleted upward. Most compatible elements are depleted throughout, relative to PAAS. Sc, however, is relatively enriched, producing a general decline in Th/Sc upward. With the exception of a single sample at the base of the sequence, ∑REE shows little variation from Upper Permian to Smithian, ranging from 159 to 146, but increases to 206 in the Spathian. Chondrite-normalised La N /Yb N also increases upsection, from ∼9.0 to 12.8. Eu/Eu* values are 0.48–0.54 in the Upper Permian–Dienerian sequence, but anomalies become smaller (0.53–0.65) and more similar to PAAS (0.66) in the Smithian and Spathian. Such values are typical of continentally derived sediments, but the slight decrease upward suggests a small influx of more mafic material. This, and decreasing Th/Sc, contrasts with increasing La N /Yb N and ∑REE, which imply an increasingly felsic source. These anomalies may be resolved by modeling potential sources using the REE and Th/Sc. The modeling suggests that the Permian–Dienerian sediments could have been derived from a source of Upper Proterozoic crust (UPC), Phanerozoic granite (PG) and amphibolite (AMP) in ratios around 60:38:2. In the Smithian and Spathian, proportions change to 76:20:4 and 68:28:4, respectively. Although the increase in the AMP component is small, when combined with increase in UPC at the expense of PG, it is sufficient to approximate the changes in Th/Sc and Eu/Eu*. La N /Yb N ratios in the Permian–Dienerian may be influenced by concentrations of HREE-enriched garnet and zircon. CIA indices and A–CN–K relations show that source weathering conditions oscillated between non-steady state (Permian, Dienerian, Smithian) and steady state (Griesbachian, Spathian), indicative of episodic uplift at the Gondwana margin. CIA ratios in the Spathian are greatest in the Spathian, indicative of more intense weathering at that time. However, A–CN–K relations, ∑REE contents and the modeled UPC:PG:AMP mix in the Spathian could also have been produced by recycling of Smithian sediments.