The Petrogenesis And Geological Implications Of The Xialan Gabbroic Intrusion And Wengong A-type Granitic Pluton In The Eastern Nanling Range

The South China Block（SCB） is surrounded by the paleo-Pacific dynamic domain to the east and the paleo-Tethyan dynamic domain to the west. The tectonic transition between these two dynamic domains is still a matter of debate, and the Nanling Range is a crucial zone to study this tectonic transition. In addition, the igneous acativities at 205-180 Ma were related to this tectonic transition, but they were insignificant in the southeastern SCB. Thus, the identification and systematical studies of the Early Jurassic igneous rocks are crucial to constrain the initial timing of this tectonic transition and the tectonic setting of the Early Jurassic in the southeastern SCB. Recently, a few Early Jurassic intrusions have been identified in the eastern Nanling Range, but the petrogenesis and tectonic implications of these Early Jurassic intrusions still remains highly controversial. In this paper, a comprehensive analysis of zircon U-Pb geochronology, in-situ zircon Lu-Hf isotope and whole-rock major and trace elements as well as Sr-Nd isotope was conducted with the Wengong granite and Xialan gabbro. These data, combined with available geological observations, provide an important insight into the initial timing of the tectonic transition and the Mesozoic tectonic evolution of the southeastern SCB. The main advances are listed as following.1) The Wengong pluton and Xialan intrusion are located in the eastern Nanling Range. The former is composed of biotite-hornblende granite, and the later consists of hornblende gabbros and olivine gabbros. The Wengong pluton yields the LA-ICP-MS zircon U-Pb age of 196.9±4.4 Ma, with the corresponding εHf（t） values ranging from +2.1 to +7.7. Olivine gabbros yield a SIMS zircon U-Pb age of 190.6±1.3 Ma, whereas hornblende gabbros give a SIMS zircon U-Pb age of 197.2±1.8 Ma with the corresponding εHf（t） values ranging from +7.0 to +12.7.2) Olivine gabbros show a restricted composition with Si O₂ of 47.11-48.75 wt.%, Al2O3 of 10.84-16.64 wt.%, mg-numbers of 75-76, Cr of 86.7-112 ppm and Ni of 63.2-114 ppm. The samples are depleted in LILE and LREE with significant negative Nb-Ta and Th-U anomalies, moderated negative Ti and strong positive Sr anomalies, which are similar to the N-MORB. Compared with olivine gabbros, hornblende gabbros show a wide compositional range with Si O₂ of 47.14-56.13 wt.%, Al2O3 of 14.58-16.25 wt.%, mg-numbers of 47-67, Cr of 27.7-353 ppm and Ni of 27.0-136 ppm. The samples are characterized by enrichment of LREE and highly incompatible elements, insignificant negative Nb-Ta and Ti anomalies, and variable Eu, Sr anomalies. Both hornblende gabbros and olivine gabbros show positive εNd（t） values ranging from +0.82 to +2.89 and +1.75 to +2.18, respectively. These geochemical and isotopic characteristics imply that olivine gabbros originated from an ancient metasomatised lithosphere mantle, whereas hornblende gabbros resulted from the mixture of asthenosphere mantle-derived magma and the parental magma of olivine gabbros. They were associated with an intra-plate extensional setting rather than the plaeo-Pacific subduction system.3) The Wengong pluton shows high Si O₂（69.22-76.42 wt.%）, K2O（3.81-4.59 wt.%）, Fe Ot/Mg O（4.40-9.75）, 10000 Ga/Al（2.86-3.33） and low Ca O（0.17-1.96 wt.%）, Mg O（0.17-0.82 wt.%） and P2O5（0.02-0.08 wt.%）, similar to A2-type granite. The samples are characterized by steep REE normalized patterns and depleted in Nb-Ta, Zr-Hf, Ba, Sr, Ti and Eu. Their initial 87Sr/86 Sr ratios range from 0.70751 to 0.71046 and εNd（t） values from-2.9 to-0.5, similar to those of the Early Paleozoic mafic rocks in the southeastern SCB. The geochemical and isotopic characteristics imply that the Wengong A-type granite was derived from the partial melting of the mafic rocks which underplated into the crust during the Early Paleozoic.4) The formation ages of the Mesozoic A-type granitic plutons in the southeastern SCB can be grouped into 229-215 Ma（Late Triassic）, 197-156 Ma（Jurassic） and 135-93 Ma（Cretaceous）. The temporal-spatial pattern and geochemical characteristics of these three groups A-type granites are distinct from each other. Available data indicate that the Late Triassic A-type granites are geodynamically linked to the paleo-Tethyan tectonic domain, and the Cretaceous A-type granites to the paleo-Pacific tectonic domain. The Jurassic A-type granites formed in an intra-plate extensional setting in response to the tectonic transition from the paleo-Tethyan to paleo-Pacific tectonic domain. The tectonic transition in the southeastern SCB might have initiated at the Early Jurassic.