| 其他摘要 | The geodynamics of late Mesozoic in SE China have been received considerable attention by lots of geologists, especially for the continental lithosphere and large scale magmatism. It is of advantage to research the geodynamics of SE China for the distributing mafic dikes. A number of works including chronology, mineralogy petrologic and geochemistry had been undertook in detail with the mafic rocks sampled in South China (Fujian, Guangdong, Jiangxi, Hainan, Hunan provinces et al.), and lots of issues had been made.
The Gan-Hang tectonic belt locates at the joint of Yangtze Block and Cathaysia Block spreads from Jiangnan Proterozoic Island arc to South China Caledonian orogeny that both belong to secondary tectonic units. It had long-term controlled the geological tectonic, magmatic activity, sedimentary, metamorphic and mineralization in both sides of the tectonic belt. It provides important chance to understand tectonic belt activity with research in the mafic rocks. But few of the mafic rocks located in the tectonic belt are referred. Thus, the paper focuses on systematic mineralogy, element and isotope geochemistry and isotope chronology of the mafic intrusions and dikes in Gan-Hang tectonic belt, and on the base of conclusion of emplacement time and source characteristics, preliminary discuss the background of geodynamics including the lithospheric thinning and extension, mantle characteristics et al. The following conclusions have been drawn:
1) According to the field and petrologic characteristics and building upon previous works collected, the mafic rocks of Gan-Hang tectonic belt are mainly composed of gabbros, diabases and olivine diabases, distributed in the south of the tectonic belt and controlled by its primary and secondary faults.
2) The mafic rock bodies from the Gan-Hang tectonic belt are mainly gabbros, belonging to sub-alkaline and alkaline series. The SiO2 content of the mafic bodies from Gan-Hang tectonic belt ranges 45.11-53.47 wt%, the MgO ranges 4.06-9.28 wt% and TiO2 ranges 0.79-3.63 wt%. The trace elements of the mafic bodies are characterized by enrichment in LREE and LILE (Ba, Rb) and depletion in HREE and HFSE (Ta, Nb, Zr, Hf and Ti). The mafic bodies from Yujiang, Dongxiang, Jiantou and Hutou where close to the centre of belt don’t have obvious Nb and Ta anomalies and their source are similar to OIB mantle. The mantle characteristics of magma source range from depleted to enriched mantle, mantle fluid, melts and involved lower crust may play an important role in the mantle enrichment process. Mafic bodies were mainly derived from partial melting and didn’t undergo obvious crustal contamination. There occurred fractional crystallization of olivine and clinopyroxenes when the magma ascent. The Gan-Hang tectonic belt not only played an important role in magma source and deep crustal-mantle interaction, but also controlled the magmatic intrusion.
3) The mafic dikes from Gan-Hang tectonic belt are mainly diabases, belong to sub-alkaline and alkaline series. The SiO2 contents of the mafic dikes ranges are 44.44-54.73 wt%, the MgO ranges are 2.74-7.89 wt% and TiO2 ranges are 0.91-3.39 wt%. The trace elements of the mafic dikes are characterized by enrichment in LREE and LILE (Ba, Rb) and depletion in HREE and HFSE (Ta, Nb, Zr, Hf and Ti). Mafic dikes were mainly derived from partial melting and didn’t undergo obvious crustal contamination. There occurred fractional crystallization of olivine, clinopyroxenes and little plagioclase when the magma ascent. Fluid, melsts and involved lower crust may play an important role in the process of the mantle enrichment. A little lower crust delaminated into primitive mantle and the metasomatism of fluid both result in the enrichment of primitive mantle. Gradual increasing of lithospheric extension and asthenosphere upwelling provided advantage for the delaminated lowser crust. The Gan-Hang tectonic belt played an important role in magma source, deep crustal-mantle interaction and the magma intrusion.
4) Mafic rocks from Gan-Hang tectonic belt have multiple characteristics on their development, and Sr-Nd-Pb isotope characteristics show the participation of EMⅡ-type enriched mantle. Complex characteristics of the fluid indicate that mantle source had been metasomatised by phlogopite and rutile, being a farther support of participancy of lower crust in the interaction with the mantle. The isotopic simulations indicate that old basement also participates through mixed in the magma origin. Combinedwith previous studies, the genetic model is provided. The Gan-Hang tectonic belt had undergone lithospheric thinning and asthenosphere upwelling with gradual increasing of lithospheric extension and the structural constraints is very important. The Cenozoic lithosphere mantle inherited and reconstructed the Mesozoic lithosphere mantle in the mantle evolution.
5) Based on the K-Ar dating and previous chronologic data, mafic rocks from Gan-Hang tectonic belt are periodically distributed. They are primarily divided into five periods; including ca. 180 Ma, 145-150 Ma, 120-140 Ma, 95-110 Ma and 65-80 Ma. Two periods in 120-140 Ma and 95-110 Ma are remarkable, may be the two forceful times of magmatism. Up to now studying, 145 Ma have set a lower limit on the start time of lithospheric extension in SE China, and the earlier accurate ages that indicate transition of tectonic activity may be turn up going with more study. Older than 140 Ma magmas are partial to the result of structure-magma activity in the background of tectonic system transform, and are related with the lithospheric thinning and asthenosphere upwelling. Entire South China lithosphere extension is intimately related to large-scale metallogeny. Gan-Hang tectonic belt give the advantaged condition to mineralization in heat source, origin of fluid and driving mechanism. |
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