Coupling of P-T-t-D histories of eclogite and metagreywacke- Insights to late Ordovician-Silurian crustal folding events recorded in the Beishan Orogen (NW China)

摘要

The Beishan complex is composed of orthogneiss and metagreywacke that both enclose bodies of eclogite and serves as a unique example for comparative petrological study of all these lithologies. The rocks show the earliest regional steep N-S striking fabric (S2) preserved in low strain domains that are reworked by ubiquitous steep N-NE dipping cleavage (S3). The eclogite shows an almost isotropic fabric defined by an M1 assemblage of Grt-Cpx-Amp-Qz-Rt-Ilm that is locally retrogressed to M2-3 amphibolite facies mineral assemblages, with P-T peak at 20-21 kbar and 750-775°C and retrogression to 2-3 kbar and 530-550°C. The typical mineral assemblage of the host metagreywackes is Bt-Ms-Pl-Qz-Chl-Ilm±Grt. Rare Al-rich metagreywacke layers are composed of Grt-Ky-St±Sil-And-Bt-Ms-Pl- Qz±Chl±Rt-Ilm giving a P-T path with peak at 8-8.5 kbar and ~670°C correlated with the S2 fabric and retrogression to ~2.5 kbar and 525-550°C correlated with the S3 foliation. In two eclogite samples, the garnet-whole rock-clinopyroxene Lu-Hf isochrons give ages of 461.9±1.6 Ma and 462.0±6.2 Ma interpreted as reflecting average age of garnet formation, and Sm-Nd isochrons give ages of 453.6±2.7 Ma and 452.8±3.0 Ma interpreted as dating near-peak metamorphism. In metagreywacke, in-situ U-Pb dating of monazite gives two groups of ages of 445-440 Ma (Mnz cores) and 436-429 Ma (Mnz rims), interpreted as reflecting the metamorphic peak and retrogression. Our results show that eclogite was formed during Ordovician by subduction of a continental crust (D1). Eclogite and metagreywacke underwent partly decoupled P-T-t-D paths until their juxtaposition at mid-crustal levels during a first late Ordovician-early Silurian D2 shortening. Coupling of their P-T-t-D paths occurred during exhumation in the Silurian and a second and orthogonal D3 shortening event. The data from the Beishan Orogen are consistent with a collisional intra-Gondwanan orogen located south of the Central Asian Orogenic Belt.