| 其他摘要 | The southeastern China locates at the joint of the Pacific plate and Eurasian plate. The special geological location and stmctural movements have received considerable attention by geologists. The area has experienced complicated and various structural movements since the Indosinian stage, of which the most important thing is the massive Yanshanian structural-magmatic event. Recently, two important things need to be further addressed: (1) The time of the transition from compressional to extensional setting, and the geodynamics processes in SE China; (2) The relationship between polymetailic mineralization and lithospheric thinning and crustal extension. Thus, the paper focuses on mineralogy, petrology, petrogeochemistry, isotopic chorology, and Sr-Nd-Pb isotopes of the mafic intrusions and dikes in Fujian province, SE China. In addition, we also discussed the process of the plate suhduction, mantle evolution, crust-mantle interaction, lithospheric thinning and extension, and the reaction for crustal extension in Fujian province, SE China. Daiqianshan mafic intrusion is situated along the Changle-Nanao fault, which intruded in the metamorphic rocks. The metamorphic rocks are composed of biotite geniss, amphibole schist, sillmanite schist and Mesozoic volcanic rocks. This paper indicates that the island affinity of the intrusion is related to subduction. However, the age of the rock closely approximates the timing of low-angle oblique subduction of an ancient Pacific plate, high-pressure metamorphic event, uplift of the Pingtan-Dongshan metamorphic belt and movement along the Changle-Nanao shear zone. The intrusion was formed by low-degree partial melting of the upper mantle. Thus, the formation of the rock is related to lithospheric extension or collision events, which was derived from upper mantle and intruded along Changle-nanao frature zone. The Daiqianshan intrusion has the same source with other mafic rocks exposed in southeastern China, which had been experienced metasomatism before emplacement. But the former has not been subjected to significant crustal assimilation. Major- and trace-element, as well as Sr-Nd-Pb isotopic data of the Mesozoic Maopin and Shaianjiao mafic intrusions, exposed in Nanjin region, Fujian province, provide an insight into the nature of their mantle source and the secular evolution of the lithospheric mantle beneath the Cathaysian Block, SE China. They are characterized by higher AI2O3, CaO and MnO contents, and lower FeOx, MgO, TiO2 and K2O+Na2O contents than those of the Cenozoic basalts. Those rocks are enriched in LILE and LREE, and relatively low HFSE, with positive Pb and negative Ti anomalies. The mafic rocks have high ( Sr/ Sr ); (0.7046-0.7077) and 207Pb/204Pb (15.47-15.67), but relatively low in (143Nd/144Nd)i (0.5125-0.5127) and 206Pb/204Pb (18.26-18.52). The negative correlation between 143Nd/144Nd and 206Pb/204Pb and the positive relationship between 87Sr/86Sr-206Pb/204Pb suggest a. mixing of a depleted mantle source and an EM2 component in the study area. Calculation reveals that the Maopin-Shaianjiao mafic rocks are formed by 5-15% degree of partial melting of an LREE-riched spinel lherzolite. The high Nb and Zr contents, as well as EM2 signature, may have been resulted from addition about 1% melts derived from subducted sediments. Their island arc characteristics suggest that the fluid derived from subducted slab still played important role in the magma genesis. The degree of the crustal contamination is about 10%, which significantly influenced their Sr and Pb isotopes. The mafic dikes in Fujian province are alkali to subalkali in compositions and are characterized by higher A12O3 (14.0-20.4 wt%) and moderate CaO (4.09-12.7 wt%) compared with the Cenozoic basalts in the same region. They are divided into two types based on their REE contents. Type 1 has low total REE (53.8-145.5p,g/g) concentration with relatively flat chondrite-normalized patterns [(La/Yb)n=1.68-4.65], whereas type 2 has much higher total REE (63-247^tg/g) concentration with LREE enrichment [(La/Yb)n=4.63-19]. In the primitive mantle-normalized spider diagrams of the type 1 dikes show positive Pb and negative Ti anomalies, with no Nb-Ta depletion, whereas type 2 dikes are enriched in LREE and LILE, and depleted in HFSE (Nb and Ta), with positive Pb and negative Ti anomalies. Zr/Y ratios correlate well with Y in the type 1 dikes but not obvious in the type 2. Rocks in both groups had experienced modest crustal contamination. The mantel source of the type 1 might have not been contaminated by subduction components (fluid+sediment), whereas that of the type 2 dikes had been modified by subducted materials (fluid + sediment). It is suggested that the type 1 dikes were derived from garnet-spinel lherzolite, whereas the type 2 dikes were derived from spinel- or spinel-plagioclase lherzolite. Both mantle sources were compositionally heterogeneous. Asthenospheric upwelling provides a possible mechanism
for lithospheric thinning and extension, leading to emplacement of the dikes. The mafic dikes have different Sr-Nd isotopic compositions. The type 1 dikes have high 143Nd/144Nd ratios than the type 2 dikes. Th/Nd and Ba/La ratios reveal that the type 2 dikes may have been experienced fluid metasomatism and subducted sediment mixing, whereas the type 1 dikes have not been contaminated by subducted components. The calculation suggests that Sr-Nd isotopic signatures of the type 2 dikes were resulted form metasomatism and mixing in the mantle source, whereas those of the type 1 dikes were mainly influenced by Paleo to Meso-Proterozoic metamorphic rocks. The above conclusion is further demonstrated by EC-AFC modelling. As a result, the characteristics of the medium-mafic dikes in Fujian province were caused by both Pacific plate subduction and crustal-mantle interaction. (5) K-Ar dating for the mafic dikes from Fujian Province, SE China, place important constraints on the timing of the mafic magmatism and tectonic evolution. The K-Ar ages show that the Cretaceous magmatism in Fujian Province, SE China, occurred in five major episodes, corresponding to the major extensional episodes: 70-75Ma ±, 85 Ma ±, 105-1 lOMa, 125Ma+ and 135-140Ma. The Cretaceous mafic rocks show island-arc chemical signatures, which might be produced by subduction before emplacement. The results indicate that the extensional environment in the region started from 140 Ma, which set the lower limit of the lithospheric extension and crustal stretch. This conclusion suggests that 90 Ma does not represent the onset of extension. Most of the mafic dikes of Fujian emplaced during the late Mesozoic (<140Ma), implying that large scale extensional events took place during the late Mesozoic and the tectonic environmental transition began after 140Ma in Fujian Province, SE China. |
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