MULTISTAGE MIXING OF THE HIMALAYAN UHP ECLOGITES: GEOCHEMICAL AND GEOCHRONOLOGICAL EVIDENCE

The Himalayan ultrahigh-pressure eclogites (Kaghan valley, Pakistan) with a peak P-T conditions of 2.7-3.2 GPa and 727-786°C, representing samples from ~100 km depth were studied for major, trace elements and isotope geochemistry. Major, trace elements and isotopic signatures show tholeiitic to mildly alkaline lava origin for these eclogites. Rare earth (REE) and trace element contents are enriched by about 10 to 100 times and 5 to 70 times over normal mid ocean ridge basalts (MORB) respectively.

The 143Nd/144Nd and 176Hf/177Hf ratios for whole-rock samples range from 0.512796 to 0.512863 and 0.282844 to 0.282909 respectively. The &epsilonNd and &epsilonHf values range from 3.08 to 5.00 and 0.91 to 3.20 respectively. Due to heterogeneous nature and isotopic remobilization, the 147Sm/144Nd and 143Nd/144Nd ratios of the whole-rocks define no isochron while separated minerals (Grt, Cpx, Ep and Ms) show isochrones ranging from 64 to 38 Ma. On the other hand, the 176Lu/177Hf and 176Hf/177Hf ratios of the whole rocks define an isochron of 100±13 Ma, while the separated minerals define isochrones range from 22 to 3 Ma.

The in situ zircon U/Pb ion microprobe isotopic ratios result in 3 distinct concordant ages. The first concordant age (266.8±3.2 Ma) probably represent the initiation of volcanic activity. The second concordant age (177.2±6.7 Ma) may either suggest a second generation of volcanic rejuvenation or assimilation of the country rock during volcanic emplacement. Final stage of volcanic emplacement or preceding cooling down of lava can be inferred from the third concordant age (114±6.4). The ion microprobe in situ zircon trace element analysis show two generations of zircons. While the first generation protolith related zircons are partly zoned and highly enriched in REE and trace elements with older age, the second generation zircons have lower REE and in trace element contents, and are typical of metamorphic origin with no concordant age obtained.

The chemical heterogeneity in major elements, large variation in trace elements, mobility of Sm-Nd and Lu-Hf isotopes and inconsistent geochronological results suggest the involvement of sediment mixing during the periodical magmatic activities and frequent element mobilization during continent-continent collision and ensuing subduction and related metamorphic/exhumation processes.