Geochemical and Sr-Nd isotopic characterisation of Higher Himalayan eclogites (and associated metabasites)

Abstract

Eclogites of the Higher Himalaya were analysed for their bulk rock geochemistry and Sr-Nd isotopic composition. These results are compared to lower grade metamorphic equivalents of rocks of the same age as weil as to unmetamorphosed basalt. Major and trace element diagrams indicate tholeiitic to alkalic magma compositions wh ich evolved by fractional crystallisation. 87 Srf 6 Sr initial values for the eclogite are 0.7053-0.7073 with values increasing for metabasites of lower metamorphic grade. 1 4 7 Sm/ 44 Nd values range from lower than chondritic to slightly higher. 1 4 3 Nd/ 44 Nd measured values range from 0.51201 to 0.5 1283, with tNd(l) from +4.9 to-10. The lowest values occur in the least metamorphosed rocks. Based on their bulk geochemistry and isotope data, the eclogites could be the equivalents of the Permian Panjal Traps. The observed isotopic "trends" are here suggested not to be explained by variations of metamorphic grade, but indicate some protoIith contamination with the evolved Higher Himalayan continental crus!.

Sm-Nd and Lu-Hf Isotope Geochemistry of the Himalayan High- and Ultrahigh-Pressure Eclogites, Kaghan Valley, Pakistan

“…The Rb-Sr isotope results (Spencer et al, 1995) and Sm-Nd isotopic results (this study) from the whole rock did not yield isochron, however, in situ zircon U-Pb isotope yielded age values between 254 and 267 Ma (Spencer & Gebauer 1996;Parrish et al, 2006;Rehman et al, 2010;Wilke et al, 2010a) show evidence of magmatic activity in Permian. From the available reference data and the present results of multi-isotope systems, we interpret that the Himalayan eclogites were originated from a wide range magmatic activity throughout the Himalayan region in the Permian to Carboniferous time.…”

“…The Kaghan Valley eclogites display large variations in major and trace element geochemistry (Rehman et al, 2008), and are in partial disequilibrium; for example Rb-Sr (Spencer et al, 1995) and Sm-Nd (Samples Ph381 and Ph422; this study). In the work of Spencer et al (1995;Fig. 4, p. 98), it is clear that the analyzed eclogite samples align poorly along the reference isochron of 270 Ma.…”

“…Patriat & Achache, 1984;Beck et al, 1995;Guillot et al, 2003). This subduction and related metamorphism resulted in a large-scale thrusting and piling up of the oceanic crust and sediments in the form of the Lesser Himalayan sequence, the Higher Himalayan crystalline and the Tethyan sediments as well as the ophiolitic mélanges along the suture zone between the Indian Plate and the Kohistan island arc (Greco et al, 1989;Spencer et al, 1995;Rehman et al, 2007) (Fig. 2).…”

“…The map is modified after Kaneko et al (2003). Spencer et al, 1995). These basaltic extrusions at the northern margin of the Indian Plate mark start of the Himalayan cycle (Honegger et al, 1982).…”

“…Greco et al, 1989;Pognante & Spencer, 1991;Tonarini et al, 1993;Spencer et al, 1995;Lombardo & Rolfo, 2000;O'Brien et al, 2001;Kaneko et al, 2003;Parrish et al, 2006;Rehman et al, 2008). Spencer et al (1995) carried out geochemical work (major and trace elements and Sr isotope) on the Kaghan Valley whole rock eclogites. They interpreted based on their geochemical results that the protolith for these eclogites was the Permian Panjal Trap basalts.…”