| The late Mesozoic geology in South China is characterized by widespread granitic intrusive-volcanic rocks and giant ore deposits, such as the famous large granitic province and (super) large ore deposit. A large number of relevant researches have been published, however, the late Mesozoic tectonic attribution and magmatic evolution in South China remain highly controversial. In addition, number of study indicate that the springs are closely associated with granitoids. The study of geothermal systems mainly concentrated in hydrology and geophysics, extremely lack of the relationship study between granitic petrology and geothermal system. The subject take the late Mesozoic typical pluton in Guangdong province as the main research object, based on comprehensive analyses of field geology, petrography, elemental chemistry, U-Pb geochronology, Sr-Nd-Hf isotopic geochemistry, combining with regional stratigraphy and regional structure to explore the time and space distribution, petrogenesis, evolution mechanism and the geodynamic background of the late Mesozoic granitic rocks in South China. Through studies of late Mesozoic magmatic activity, radiation heating rate and the geothermal heat reserves in South China to discuss the relationship between forming mechanism of the geothermal systems and the late Mesozoic granites.Through the systems analysis of petrography, zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotope geochemistry, this study yielded the main results as follow:(1) The early Jurassic Tiandong monzogranites have been discovered in Guangdong province, SE China. LA-ICP-MS zircon U-Pb dating results show that the Tiandong monzogranite has a crystallization age of 188Ma. The Tiandong monzogranites display I-type characteristics with high Sr/Y, Nb/Ta, FeO’/MgO ratios, low P2O5, lowe Fe2O3t, MgO, MnO, TiO2, CaO. It depleted in Ba, Nb, Sr, P, Ti, Zr, and enriched in Rb, Th, U, K, Pb and significantly negative Eu anomalies. The isotopic data of the Tiandong granites display relatively depleted values, with ISr=0.7032,εNd(t)=1.09 and εHf(t) from 9.1 to 13.3, average value is 11.2. The obvious Nd-Hf decoupling has been observed and Hf isotopic composition is uniform. These geochemical features suggested that the pluton was originated from a juvenile basaltic crust instead of crust-mantle mixing. The juvenile basaltic crust origined from mixing between asthenosphere melt and enriched lithospheric mantle melt. The Paleo-Pacific subduction started in the early Jurassic. The NW-trend subduction of the Pacific prompted Nanling preexisting nearly E-W trending indosinian faults to reactivate and discrete and localized intraplate extensions, which may allow asthenospheric mantle upwelling and decompression. A juvenile crust was generated by the mixing between enriched lithospheric mantle melt and asthenospheric mantle melt.(2) The Fengshun Batholith is a multi-stage magmatic intrusion, It is composed of two main units, i.e., the Mantoushan syeno-monzogranites, alkali feldspar granites (MTS) and the Hulutian alkali feldspar granites (HLT). LA-ICPMS zircon dating shows that the units are emplaced in 166-161Ma and 139±2Ma, respectively. Geochemically, the MTS granites show relatively various geochemical composition with low REE contents (87.76-249.71), Rb/Sr ratios (1.19-58.93), pronounced Eu negative anomaly (0.01-0.37) and low Nb/Ta ratios (2.40-6.82). In contrast, the HLT granites exhibit relatively stable geochemical characteristics with high REE contents (147.35-282.17), Rb/Sr ratios (2.05-10.30) and relatively high Nb/Ta ratios (4.45-13.00). The isotopic data of the MTS granites display relatively enriched values, with ISr varying from 0.7082 to 0.7097, εNd(t) from -7.8 to -6.9 and εHf(t) from -7.4 to -3.2, in comparison with those of the HLT which are ISr=0.70305-0.70477, εNd(t)=(-5)-(-3.4) and εHf(t)=(-0.7)-(+1.8). The two-stage model ages of the MTS granites (T2DM(Nd)=1.51-1.59Ga and T2DM(Hf)=1.26-1.48Ga) are also higher than those of the HLT granites (T2DM(Nd)=1.21-1.34Ga and T2DM(Hf)= 0.96-1.10Ga). Thus the MTS and HLT granites might originate from different sources. The former is more likely derived from partial melting of Meso-proterozoic basement triggered by upwelling of asthenosphere and/or underplate of the basaltic magma and then extensive fractional crystallisation, similar to the genesis of Early Yanshanian granitoids of the EW-trending tectono-magmatism-metallogenic belt in the Nanling range. In comparison, the latter might have involved with asthenosphere component, similar to the Early Cretaceous granitoids of NE-NNE-trending granitoid-volcanic belt in coastal region, Southeastern China. We proposed that the MTS granites were mainly formed in withinplate extensional tectonic setting whereas the HLT granites were formed in the back-arc extensional tectonic setting. The period at 139 Ma represented the initial time of roll-back of the paleo-pacific plate in SE-trending. The mineralization such as W, Sn was likely a result of revolution of granitic magma.(3) The Xinzhou complex, located in the southwestern coastal areas in South China, consist of medium-course grain biotite syenites/monzogranites in Xinzhou, porphyritic quartz monzogranites/syenites with medium-fine grains in Dongping and medium-fine grain biotite alkali feldspar granites in Naqin, and they formed in ～150Ma,～140Ma and ～117Ma, respectively. The Xinzhou and Naqin granites are K-rich calc-alkaline rocks, while the Dongping plutons are shoshonites. For the REEs and trace elements, all the three plutons display negative Eu anomalies, and Naqin granites have remarkable negative Eu anomalies, δEu ranging from 0.01 to 0.06 with an average value of 0.02. In the REE diagrams, both Xinzhou and Dongping plutons show V-like pattern with right deviation. However, the Naqin pluton displays a relatively flat pattern with a V-like feature, suggesting a slight differentiation of LREE and HREE (LREE/HREE=1.61-2.27, mean 1.97). Moreover, the Naqin pluton presents obvious tetrad effect in the REE diagrams (TE1, 3=1.04-1.17, average=1.10), and Xinzhou pluton has insignificant tetrad effect, Dongping pluton has no tetrad effect. In the trace element spidergrams, both Xinzhou and Dongping plutons have distinct negative Nb, Ta anomalies, while Naqin pluton has positive Nb, Ta anomalies. Furthermore, all these three plutons are characterized by the negative Ba, Sr, P and Ti anomalies, among which Naqin pluton displays the most distinct anomalies. Finally, Dongping pluton has the highest Nb/Ta and Zr/Hf ratios (Nb/Ta=14.38-18.51, Zr/Hf=44.64-48.10) and Naqin pluton has the lowest Nb/Ta (2.87-19.36), Zr/Hf (10.62-23.64) ratios, the Xinzhou pluton has an intermediate value, which suggests a crustal origin. For the isotopic compositions, Dongping and Naqin plutons have similar Sr-Nd isotopic features, while Xinzhou granites have relatively lower εNd(t) ((-5.79)-(-5.89)) than those two. In addition, Xinzhou granitic pluton has lower Hf isotopic composition with εHf(t) ranging from -6.3 to -3.9 than the Dongping pluton (εHf(t)=(-2.2)～(+1.3)). Thus, the Xinzhou granites should form in an intra-plate setting instead of a subduction-related active continental margin. The Dongping syenite pluton with a typical shoshonitic feature should be the products mixed by metasomatic enriched mantle and asthenospheric materials under the lithospheric extensional setting. Finally, the Dongqin pluton should be resulted from the extension of lithosphere, independent from plate subduction.(4) An evolution model of the tectono-magmatism-mineralization in South China is proposed as follows:First, since the early Jurassic (-200Ma-175Ma), Paleo-pacific plate subducted mechanism launched, when there was none of the volcanic rock formation. Subduction stress leads to reactivation of the preexisted indosinian E-W fault zone and then produce the early Jurassic E-W intraplate magmatic belt in South China. Second, a strong flat-slab subduction or very low angle subduction happened in middle Jurassic to early Cretaceous period (～175-140 Ma). Subduction accompanied by intense stress resulted in further extensions of the E-W fault zone in Nanling and adjacent regions and a large number of asthenosphere material upwelling and then gave rise to a large-scale crust remelting to develop a large granitic province (the first magmatic explosion) and large-super large ore deposit in south China, and other regions developed compressive structure. Third, the roll-back of the paleo-pacific plate was started since ～140-120 Ma, in resposed to which extensive volcanic stratas exposed along the SE coast area and magma erupted in South China (the second explosive magmatism). Fourth, Collision between South China plate and West Philippine plate during 120-110 Ma, which produced a magmatic "quiescence". Fifth, Lithosphere extension since 110 Ma, especially ～93 Ma, led to the third magmatic explosion in South China. Crust suffered significant extrusion, uplift and denudation in-100 Ma.(5) Most of high heat producing granites (HHP granites) are metaluminous to weakly peraluminous with ACNK<1.1, most varying from 1.0 to 1.1 and ANK=1.0-1.3. Xinzhou complex contains a heat production of 5.58×10 J/a, and the value is equal to 1.55×108kwh per year or 19,000 tons standard coal. The middle-high temperature hot springs are usually discovered nearby or in the margin of the large batholith, which imply a close relationship with the scale and radioactive heating rate of batholiths. There is a good corresponding relation between the radioactive heating peak and the explosion of magmatism in South China. It suggests that the large-scale remelting of the crust material prompted the radioactive elements such U, Th, K enrichment effected by the mantle material and thermal. Such process is the main factor of the heat production instead of the concentration differences of radioactive elements for basement rocks. There may not be an effective magma chamber (or capsule) under South China continent. The late Mesozoic granites are the primary heat source in upper crust. |
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