Search

Subduction and collision are the key processes triggering geochemical refertilization of the lithospheric mantle beneath cratons. However, the way that the subducted plate influences the cratonic lithospheric mantle remains unclear. Here, in-situ major and trace-element and Sr isotopic compositions of peridotite and pyroxenite xenoliths carried by the Dongbahao Cenozoic basalts, located close to the northern margin of North China Craton (NCC), were examined to investigate the effects of the subducted Paleo-Asian oceanic plate on the lithospheric mantle of the NCC. Based on petrographic and geochemical features, peridotites were subdivided into two types recording two-stage metasomatism. Clinopyroxene (Cpx) in both types of peridotites show chemical zoning. In those peridotites we refer to as Type 1 peridotites, Cpx exhibit uniform convex-upward rare earth element (REE) patterns but core-rim variations in 87 Sr/ 86 Sr ratios (0.7065–0.7082 in the cores and 0.7043–0.7059 in the spongy rims), and have high (La/Yb) N ratios (> 1.12) (N means normalized to chondrite), relatively low Ti/Eu ratios (< 3756) and negative high field strength element (HFSE) (Nb, Ta, Zr, Hf and Ti) anomalies in the cores, indicating early-stage metasomatism by carbonatitic melts derived from the subducted sedimentary carbonate rocks. Cpx in the Type 2 peridotites have highly variable REE patterns (from light rare earth element (LREE)-depleted to LREE-enriched) and feature zoned Sr isotopic compositions contrasting to those in Type 1, i.e., increasing 87 Sr/ 86 Sr ratios from the cores (0.7020–0.7031) to the spongy rims (0.7035–0.7041). Accompanying variations of 87 Sr/ 86 Sr ratios, Cpx in both types of peridotites display increasing Nb/La ratios from the cores to the spongy rims. In addition, Cpx in the Type 2 peridotites show remarkably increased (La/Yb) N , Ca/Al, Sm/Hf and Zr/Hf ratios but decreased Ti/Eu and Ti/Nb ratios from the cores to the spongy rims. These features imply a later-stage metasomatism by CO 2 -rich silicate melts derived from carbonated eclogites. Pyroxenites were also classified into two types. Both types of pyroxenites show higher Ni content in Cpx and orthopyroxene than peridotites at the same Mg# (= 100 ∗ Mg/(Mg + Fe), atomic number) level. Their Cpx show high Ti/Eu, Ti/Sr ratios and similar 87 Sr/ 86 Sr ratios (0.7039–0.7055) to the Cpx spongy rims in peridotites, suggesting that pyroxenites originated from silicate melt-peridotite reactions in the later-stage metasomatism. These observations collectively indicate that the lithospheric mantle beneath the northern NCC presents evidence for two distinct mantle metasomatic events. We propose that both were caused by the subduction of the Paleo-Asian oceanic plate, which could have contributed significantly to the transformation of the lithospheric mantle beneath the northern NCC. [ABSTRACT FROM AUTHOR]

Palaeoarchaean granitoids are exposed over wide area in the Singhbhum craton whose origin and role in crustal evolution are not well constrained. This study presents whole-rock and mineral chemical data coupled with zircon U-Pb dating and Hf isotope results on such granitoids from the central part of the craton to understand their petrogenesis, tectonic setting and role in continental crustal evolution. The first phase of granitoid magmatism in this area is represented by a 3.47 Ga Na-rich, gneissic tonalite belonging to the Archaean TTG (tonalite-trondhjemite-granodiorite) suite. This rock is characterized by high-HREE (heavy rare earth elements), negative Eu anomaly, low Sr/Y ratio and positive zircon Hf isotope signature (εHf t = +2.1 to +4.8). It is interpreted to be formed by shallow melting of a juvenile mafic source. At 3.35 Ga a silicic, ferroan porphyritic biotite granite formed. It shows variable K/Na, low Y and high Sr/Y, moderately fractionated HREE and positive zircon εHf t (+1.8 to +4.0), and is explained as a product of high-temperature melting of a heterogeneous, juvenile source consisting of tonalites and mafic rocks at lower crustal depth. The final phase of granitoid magmatism is marked by a 3.30 Ga non-porphyritic ferroan, silica-rich biotite granite. Geochemical characteristics like moderately high K, moderately fractionated HREE, low Ca and Sr/Y, and zircon εHf t (+0.8 to +3.7) suggest that the granite was formed by high-pressure melting of a tonalite-dominated source with short crustal residence. All the three granitoid phases display low Mg, Ni and Cr contents and magnesium number (Mg#) precluding direct involvement of mantle in their origin. Rather, crustal reworking caused by episodic plume-related mafic-ultramafic magma underplating and intraplating in an oceanic plateau setting is suggested as the possible mechanism for formation of the granitoids. Successive crustal reworking events involved progressively greater amount of previously formed felsic crust generating more evolved, K-rich granitoids. It appears to be a globally important process which led to effective crustal differentiation and maturing of the cratons during Palaeoarchaean. [ABSTRACT FROM AUTHOR]