Huichuan Liu

Petrogenesis and tectonic implications of Late-Triassic high ɛNd(t)-ɛHf(t) granites in the Ailaoshan tectonic zone (SW China)

High ɛNd(t)-ɛHf(t) granites are robust evidence for crustal growth. In this paper we report results of petrologic, geochronological and geochemical investigations on the Huashiban granites from the Ailaoshan tectonic zone in western Yunnan (SW China). Zircon grains separated from the two samples (10HH-119A and 10HH-120A) yield the weighted mean 206Pb/238U ages of 229.9 ± 2.0 Ma and 229.3 ± 2.3 Ma, respectively, interpreted as the crystallization ages of the granites. Based on our results, in combination with the existing U-Pb geochronological data for the Ailaoshan metamorphic rocks, we propose that the Ailaoshan Group might be a rock complex composed of the Mesoproterozoic, Neoproterozoic, Hercynian, Indosinian and Himalayan components, rather than a part of the crystalline basement of the Yangtze block. The zircon grains show highly depleted Lu-Hf isotope compositions, with positive ɛHf(t) values ranging from 8.4 to 13.1. The Huashiban granites have high SiO2 (72.66 wt%–73.70 wt%), low Mg# (0.28–0.34) with A/CNK=1.01–1.05, and can be classified as peralumious high-K calc-alkaline I-type granites. A synthesis of these data indicates that the Ailaoshan tectonic zone had evolved into a post-collisional setting by the Late-Triassic (229 Ma). Genesis of the Huashiban high ɛNd(t)-ɛHf(t) granites involved into two processes: (1) underplating of the sub-arc mantle into the lower crust, and (2) remelting of the juvenile crustal materials in response to the upwelling of the asthenospheric mantle in the post-collisional setting.

Zircon U-Pb geochronology of the Cenozoic granitic mylonite along the Ailaoshan-Red river shear zone: New constraints on the timing of the sinistral shearing

The Ailaoshan-Red River (ASRR) shear zone in SW China represents an important discontinuity believed to have accommodated eastward extrusion of the Tibetan Plateau in response to the collision of the Indian and Eurasian plates. The onset timing and duration of the ASRR sinistral strike-slip shearing have been hotly disputed. In this paper we present new zircon LA-ICP-MS U-Pb geochronological data from six syntectonic granitic mylonite and leucosomes samples from the ASRR shear zone. Our data reveal a metamorphic age of ∼40 Ma, most likely suggesting the maximum age of the shearing initiation. Rocks showing syn-kinematic signatures yield crystallization ages of 38–22 Ma, with inherited components ranging from 716 to 108 Ma. These results, together with existing geological and geochronological data, indicate that the sinistral shearing along the ASRR zone probably began at 40 Ma, mainly activated at 29–22 Ma and lasted at least to ∼22 Ma. Our data suggest a continuous extrusion between the Indochina and South China blocks during ∼35–17 Ma. The ASRR sinistral shearing has accommodated large scale eastward displacement of the southeastern Tibetan syntaxis, and is likely responsible for the opening of the South China Sea.

Episodic slab rollback and back-arc extension in the Yunnan-Burma region: Insights from Cretaceous Nb-enriched and oceanic-island basalt–like mafic rocks

One highly debated topic related to the nature of the Cretaceous mantle and geodynamic processes in SE Asia is whether they were dominated by the Tethyan or Pacific Ocean subductions. The main reason for this puzzle is the lack of geochemical studies on mantle-derived mafic rocks in SE Asia. In this paper, we report Early Cretaceous Nb-enriched diabases (ca. 139 Ma) and Late Cretaceous Nb-enriched (ca. 68 Ma) and oceanic-island basalt (OIB)−like (ca. 73 Ma) amphibolites from the Ailaoshan tectonic zone in the Yunnan-Burma region. The ca. 139 Ma diabases and ca. 68 Ma amphibolites display similar geochemical features with typical Nb-enriched basalts, e.g., high Na 2 O, TiO 2 , and Nb (>7 ppm) contents, and high (Nb/Th) PM , (Nb/La) PM , and Nb/U ratios. Unlike adakite-associated Nb-enriched basalts, our Nb-enriched diabases and ampibolites were derived through mixing between OIB-like and arc-like mantle source components in the spinel stability field ( 2 and Nb (20.0−26.0 ppm) contents, and OIB-like rare earth element patterns and trace-element spidergrams. The OIB-like rocks were generated by a low degree of partial melting of garnet-facies peridotite (>80 km). In combination with previous Cretaceous to early Eocene age data from granitoids, three significant magmatic flare-ups are identified at 140−115 Ma, 75−64 Ma, and 55−50 Ma, with two magmatic gaps at 115−75 Ma and 64−55 Ma. Given the distances of the studied area from the Pacific and Neotethyan trenches (2400−5000 km and 320 km, respectively), these Cretaceous igneous rocks more likely formed in a Neotethyan subduction-related tectonic setting, rather than a Pacific Ocean−related setting. We propose a tectonic model involving episodic flat slab subduction, slab rollback, and intracontinental back-arc extension for Neotethyan subduction in the Yunnan-Burma region.

Petrogenesis of Late Silurian volcanism in SW Yunnan (China) and implications for the tectonic reconstruction of northern Gondwana

ABSTRACT Early Palaeozoic magmatic records and tectonic reconstructions along the northern margin of Gondwana are still pending problems. In this paper, Late Silurian Dawazi and Dazhonghe volcanics in SW Yunnan Province (China) were studied. The Dazhonghe volcanics (419 Ma) have variable chemical compositions with SiO2 ranging between 49.8 and 79.5 wt.%, whereas, the Dawazi volcanics (417–429 Ma) form a bimodal volcanic suite consisting mainly of silicic rocks and subordinate basaltic rocks with a SiO2 content gap of ca. 15 wt.%. The Dazhonghe volcanics display calc-alkaline elemental compositions with enrichment in light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs) (e.g. Nb, Ta and Ti) and positive εNd(t) values (+ 4.0 to + 5.5). The Dawazi basaltic rocks are calc-alkaline, depleted in HFSEs, enriched in large ion lithophile elements (LILEs) (e.g. Cs, Rb, U and K), and have high εNd(t) values of −1.7 to + 5.4. The Dawazi silicic rocks have high Na2O/K2O ratios and positive εNd(t) values of + 2.4 to + 5.0, which are equivalents of calc-alkaline I-type granites. The Dazhonghe volcanics are dominated by fractional crystallization (FC) from a calc-alkaline primary magma which originated from an enriched mantle source metasomatized by subduction-related, sediment-derived fluid. The Dawazi basaltic rocks were derived from partial melting of an enriched mantle source metasomatized by subducted oceanic sediment/slab-derived fluids; the Dawazi silicic rocks originated from partial melting of the juvenile mafic lower crust with extensive FC. Both the Dazhonghe and Dawazi volcanics were generated in a continental back-arc extension setting. Combined with previous geological observations, a Late Silurian Prototethyan arc and back-arc extension system is proposed along the northern margin of Gondwana in SW China.