Early Late Cretaceous Magmatic Activity In Central And North Lhasa Subterranes,and Their Tectonic Implication

Stratigraphic and sedimentary studies in the Nyima basin, suggested that that the Lhasa terrane has been a highstanding feature, the “Lhasaplano”, since the Late Cretaceous, which mean the crust thickening of Lhasa terrane prior to Indo-Asian collision. These studies, however, concentrated mainly on evidence from stratigraphy and sedimentary rocks, and not on contemporaneous igneous rocks. In this paper, we report on the Gaerqiong diorite porphyries and Xiongma plutons which are located in the northwestern part of the central Lhasa subterrane. New zircon U–Pb ages, geochemical analyses, and Sr–Nd–Hf isotopic studies of these Gaerqiong diorite porphyries, and Xiongma plutons are conducted, combined with previous published date on late cretaceous igneous in Lasha terrane, we intend to provide a critical framework of the late early cretaceous tectonomagmatic evolution of Lhasa terrane.(1) The Gaerqiong diorite porphyries which formed at ~84.6 Ma, have high Mg O(3.56–4.86 wt.%), Cr(>150 ppm), and Ni(>70 ppm) contents, and they show adakitic affinities such as high Sr and low Y contents, and high Sr/Y ratios. The GPs can therefore be classified as high-Mg# adakitic diorites.The GPs sampled in the northwestern part of the central Lhasa subterrane are characterized by depleted whole-rock(87Sr/86Sr(i) = 0.7051 to 0.7053, εNd(t) = +1.65 to +2.0 values and zircon εHf(t) = +5.2 to +9.9), which are contrast with the ancient central Lhasa subterrane(87Sr/86Sr(i) = 0.7402, εNd(t) =-15.4),indicating the presence of a juvenile mafic lower crust beneath this part of the central Lhasa subterrane. This juvenile lower crust was produced by mafic magma underplating that was triggered by rollback and break-off of the southward subducted Bangong-Nujiang Ocean lithosphere at 140~125 Ma and 113Ma。(2)The Xiongma pluton are mainly granites and MMEs, U-Pb zircon ages of the granites and MME yield coeval at Ca. 88 Ma. Igneous microtextures, the presence of acicular apatites and K-feldspar megacrysts of the enclaves implies that the enclaves are remnants of mafic component. The Xiongma MMEs have low Si O2 contents(48.9 to 52.76 wt.%) with relatively high Mg-numbers(Mg#=47 to 49), Mg O(4.47 to 4.65 wt.%), Fe2O3(9.52 to 9.94 wt.%), Ti O2(1.1 to 1.3 wt.%) and Cr(18.3 to 39.7 ppm) concentrations, meanwhile, they are enriched in LILEs and LREEs and depleted in HFSEs with negative Nb, Ta, Zr and Hf anomalies in the PM-normalized trace element patterns,In addition, they have enriched εHf(t)(-11.9 to-0.1) isotopic compositions. All these geochemical features, similar to the typical subduction-related rocks, indicating that their parental magmas were directly derived from an enriched mantle. The host granites belong to the I-type suite. They have really high Si O2(65.95-68.43 wt.%), low Mg O(1.17 to 1.55 wt.%), Mg#(40 to 42), Fe2O3(3.38 to 4.49 wt.%), Ti O2(0.38 to 0.53 wt.%), Cr( 6.3 to 21.7 ppm). Variable concentrations of major elements and also variable εNd(t)(-7.4 to-6.3) and zircon εHf(t)(-12.8 to-6.0) values, combined with the presence of MMEs in the Xiongma granites, we suggest Xiongma granites were not simple produced by fraction of basaltic magmas or partial melting of crust of central Lahsa terrane. They were mostly likely derived from partial melting of ancient crustal materials, and mixed with enriched mantle-derived magmas.Whole-rock Sr-Nd isotopic modeling further show that the Xiongma granites can be generated by the mixing between partial melts of 80% central Lhasa crust(50% ancient crust and 50 % juvenile crust) and 20% enriched mantle-derived magmas.(3)Two nearly ~90Ma magmatic belts were found in the Lhasa terrane(the southeastern part and northwestern part). These two magmatic belts show different spacial distribution and contain distinctive types of rocks. i.e. The ~90Ma magmatic belt in the southeastern part of Lhasa terrane distribute parallel to Indus–Yarlung Zangbo suture. The type of rocks from this belt contains both sillicic rocks(mainly granitoids), intermediate rocks(diorites) and mafic rocks(gabbros, norites and hornblendites). The granitoids show calc-alkaline characters and those mafic rocks show clear island arc basalts(IABs) affinities. Although the geodynamic mechanism(slab rollback or ridge subduction) of these rocks was in a hot debated, it reaches a consensus that those rocks are all related to subduction of Neo-Tethys plate. Thus, the ~90Ma southeastern magmatic belt was in the Neo-Tethys subduction domain. In contrast, the ~90Ma rocks of the northwestern magmatic belts distribute sporadically. The magmatic rocks of this belt contains mainly sillicic rocks and rare mafic magmas. The sillicic rocks are adakitic-like rocks and/or granites and the mafic rocks show OIB and island arc basalts(IABs) affinities. Considering the ~90Ma northwestern magmatic belt have different spatial distributions, distinctive type of rocks from the southeastern magmatic belt, combined that northwestern magmatic belt were emplaced at about 350 km to north of the Yarlung-Zangbo suture zone during the Late Cretaceous, we suggest the geodynamic regime for the northwestern magmatic belt is unlikely related with the Neo-Tethys subduction. Due to the northwestern magmatic belt are near Bangong–Nujiang suture zone, thus their geodynamic regime are perhaps related with Bangong–Nujiang oceanic evolution. Combined with previous study on northwestern part of Lhasa terrane, we suggest the crust of northwestern part of Lhasa terrane may have delaminated. The continental delamination have occured in northwestern part of Lhasa terrane at ~90Ma, which indicates northwestern part of Lhasa terrane was probably uplift at late cretaceous.