Evidence of the Collision Between Yangtze and Cathaysia Blocks

SIMS zircon U–Pb ages, geochemistry and Nd–Hf isotopes of ca. 1.0 Ga mafic dykes and volcanic rocks in the Huili area, SW China: Origin and Tectonic Significance

Authors:

Zhu et al

Abstract:

Late Mesoproterozoic to early Neoproterozoic igneous rocks (ca. 1100 Ma to greater than 860 Ma) are sparse in the southwestern Yangtze Block, hindering our understanding of South China's position during the late Mesoproterozoic to early Neoproterozoic assembly of the supercontinent Rodinia. We report here Cameca secondary ion mass spectrometry (SIMS) zircon U–Pb ages, geochemistry, and Nd–Hf isotopic data for mafic dykes that intruded in the lower Tianbaoshan Formation, and that of intermediate to felsic volcanic rocks from the upper Tianbaoshan Formation of the Huili Group in southwestern Yangtze Block. The mafic dykes were formed at 1023 ± 6.7 Ma. The volcanic rocks were dated at 1025 ± 13 Ma and 1021 ± 6.4 Ma, contemporaneous to the mafic dykes. The mafic dykes show weakly positive ɛNd(t) values (+0.41 to +1.6) and positive ɛHf(t) values (+7.0 to +10.3). They are characterized by slightly LREE-enriched and HREE-depleted patterns and the trace element patterns with typical depletion of Nb and Ta and slightly enrichment of Th relative to La. The parental magma for the mafic rocks exhibits affinity of low-Ti tholeiitic basaltic magma generated by melting of a depleted asthenospheric mantle source. The primary magma of these mafic rocks likely underwent variable degrees of the fractional crystallization and crustal contamination. The intermediate to felsic volcanic rocks in the Tianbaoshan Formation are characterized by significantly LREE-enriched and slightly HREE-depleted patterns, showing negative Nb–Ta, Eu, Sr, P and Ti anomalies in the primitive mantle-normalized multi-element plot. In addition, the volcanic rocks display variably elevated Ga, Rb, Zr, REE, and Y concentrations, and high Ga/Al ratios (2.31–2.64), consistent with the geochemical characteristics of A2-type granites. The volcanic rocks exhibit negative ɛNd(t) values (-5.1 to -7.1) and variable ɛHf(t) values (-0.67 to +3.9). They give two-stage Nd model ages (TNd2DM) and two-stage Hf model ages (THf2DM) of 1.83 to 1.99 Ga and 1.62 to 1.91 Ga, respectively. The Nd–Hf isotopic signatures indicate that the primitive magma for the Tianbaoshan intermediate to felsic volcanic rocks was probably derived from partial melting of the granulite-facies lower crust driven by underplating of mafic magmas, and underwent extensive fractional crystallization during emplacement. The studied 1.02 Ga felsic volcanic rocks and mafic dykes at Huili are interpreted to have formed in an impactogen setting during the initial stage of collision between the Yangtze and Cathaysia blocks prior to the Sibao Orogeny.

Subduction-Driven Plate Tectonics MAY Date From MesoArchean

Petrogenesis of Neoarchean TTG rocks in the Yangtze Craton and its implication for the formation of Archean TTGs

Authors:

Wu et al

Abstract:

The Yangtze Craton is one of the most important cratonic blocks in eastern Asia. However, its early evolution has not been well constrained yet, because of the poor exposure of Archean basement rocks. In this contribution, a combined study of zircon U-Pb age, Hf-O isotope compositions, as well as whole rock element compositions was carried out for gray gneiss rocks from the Douling complex in the northwestern part of the Yangtze Craton. Zircon U-Pb dating for two gneiss samples yielded weighted mean ages of 2509 ± 21 and 2496 ± 15 Ma. The gray gneisses have high SiO2 (63.64-73.74%), Na2O (3.66-6.21%), but low K2O (0.76-2.93%) contents, and belong to the trondhjemite, tonalite and granodiorite series rocks. They are characterized by enrichments of LILEs and LREEs, but notable depletions in Nb–Ta and Ti, as well as heavy REEs and Y. All the samples plot into the fields of typical Archean TTG rocks in the (La/Yb)N vs. (Yb) N and Sr/Y vs. Y diagrams. Therefore, the gray gneisses in the Douling complex are Neoarchean TTG rocks. The relatively high Nb/Ta and Zr/Sm ratios of 19.6-46.0 and 14.5-143.4 suggest their formation under eclogite-facies conditions. They have low ɛHf(t) values of -3.21 and -3.39 and Hf model ages of 3194 ± 26 and 3205 ± 24 Ma, indicating their derivation from ca. 3.2 Ga juvenile mafic rocks. Their δ18O values of 6.41 ± 0.12 and 5.44 ± 0.12 ‰ suggest that surface rocks were differently recycled into their sources. Collectively, the TTG rocks in the Douling complex were generated by partial melting of thickened mafic crust rather than direct partial melting of oceanic crust in a convergent plate boundary. A compilation of available zircon U-Pb and Hf isotope compositions of the Yangtze Craton reveals that there are quite differences of Hf isotope compositions in Archean. The ca. 3.8-3.4 Ga zircons exhibit ɛHf(t) values of -5.8 to 0.0. These chondritic to slightly enriched hafnium isotope compositions imply that their host rocks were formed by protracted intra-crustal reworking of ancient protoliths without juvenile input. By contrast, zircons with ages younger than ca. 3.2 Ga register both negative and positive ɛHf(t) values up to the depleted mantle, providing clear evidence for the addition of juvenile materials. These imply that the subduction-driven plate tectonics might have been operated since ca. 3.2 Ga in the Yangtze Craton.