The Early Cretaceous Evolution Of SE China And Its Tectonic Implications

The Mesozoic geology in SE China is characterized by widespread Jurassic-Cretaceous magmatism, which is usually considered related to the west-directed subduction of Paleo-Pacific plate. However, there appears an episode of "magmatic quiescence" at ca.130-110 Ma in terms of increasing high-precision isotopic age data. The causes and regional tectonic significances of this magmatic quiescence are unclear. In this study, we report petrography, mineral chemistry, whole rock geochemistry, and integrate in situ U-Pb, Hf and O isotope analyses of zircons from schist, gneiss and granitoid samples along Changle-Nan’ao area in SE China coast, and use these data to study the timing of the regional metamorphism, granitoid petrogenesis and their implications for the regional tectonic evolution.(1) The Xiaocuo granitoid complex consists of undeformed granites, granodiorites and diorites; all these rocks dated at-116 Ma are metaluminous to weakly peraluminous (A/CNK= 0.84-1.13). They are characterized by homogenous isotopic compositions (i.e., initial 87Sr/86Sr (Isr)= 0.7054-0.7056, εNd(T)=-1.6 to -2.6, zircon εHf(T)=-0.6 to +3.5 and zircon δ18O= 3.8-5.2‰), implying that they were derived from one common source. On a log-log plot of compatible against incompatible trace elements, Sc decreases dramatically with a slightly increase of Rb, suggesting that fractional crystallization rather than partial melting plays a major role between the rock types. In Harker diagram, these rocks display a significant decreasing of Al2O3, MgO, Fe2O3, TiO2, Sc and CaO with increasing SiO2, and granites show a significant increasing of K2O and Ba with increasing SiO2. This correlation was attributed to the amphiboles accumulation in diorites, biotite and plagioclase fractional crystallization in all the rocks, as well as accumulation of K-feldspar in granite. These rocks have high Na2O/K2O (>1), low Sr/Y (7.23-49.5) and La/Yb ratios (6.87-46.0). In addition, the diorites and granodiorites contain a variety of inherited high-Ca plagioclase (An= 60-87). Accordingly, the Xiaocuo granitoid rocks were derived from preexisting, subduction-related high-Al mafic source rocks at low pressure condition. The comparable oxygen isotopic equilibrium temperatures (730 ± 70 ℃) and Zr saturation temperatures (～730 ℃) in granite indicate that the low δ18Ozrc values (3.8-5.2‰) are reflective of their source compositions. The low-18O source rocks is most likely formed in a subduction-induced lithospheric extension regime in Jurassic when a Basin and Range-like province developed in SE China. Intensive lithospheric extension could have increased the fracture permeability and facilitated deep-circulated surface waters for exchanging with the source rocks at high temperature at depth, making the metabasic source rocks depleted in 18O. Remelting of the 18O-depleted metabasic intrusive source rocks produced the Xiaocuo low-18O granitoid complex at～116 Ma.(2) The gneisses in Dongshan area are dated between 150 and 141 Ma; the undeformed monzonitic granites are dated between ～107 and 103 Ma. All the granitic gneiss and monzonitic granites are belong to calc-alkaline to high-K calc-alkaline and weakly peraluminous to strongly peraluminous series. They have relatively low FeOT/MgO and 10000Ga/Al (<2.5) ratios, and low Zr, Nb, Y, LREE contents, which is different from typical A-type granite. In addition, the S-type granite origin can be excluded due to mantle derived zircon-similared δ18O values. In this regard, these rocks are belong to (highly fractionated) I-type granitic rocks. The plotting of SiO2 vs. K2O implies that these rocks were derived from a metabasaltic source. The zircons in gneisses have εHf(T) values ranging from -5.1 to 0.7 and δ18O values ranging from 5.0%o to 6.9‰ (mean= 5.9 ± 0.8‰ (2SD, n= 51)); the zircons in granites have εHf(T) values ranging from 0.2 to 1.6 and δ18O values ranging from 4.9%o to 5.9%o (5.3 ± 0.6‰ (2SD, n= 11)). Zircon εHf(T) value show a increase trend but δ 18O value exhibit a decrease trend from early Early Cretaceous to late Early Cretaceous time, indicating an increasing contribution of juvenile crust in their petrogenesis.(3) The majority of zircons from the schists are euhedral to subhedral crystals showing clear concentric zoning under cathodoluminescence (CL) images. The zoned zircons from all schist samples are homogeneous in U-Pb age zircon ages (between 131 and 148 Ma, peaking at 141 Ma), O isotopes (δ18O= 5.6 ± 0.8%o,2SD, n= 106), and, to the less extent, Hf isotopes (sHf(T)=-4.7 to-1.4 for the Dongshan schists, and-1.4 to 2.2 for the Putian schists). These zircons have mantle derived zircon-similared δ18O values, and the occurrence of them is not related to increasing of metamorphic grade, indicating that they are most likely detrital origin. The comparable zircon Hf isotopic characteristics implies they were derived from the regional Early Cretaceous felsic volcanic rocks (the Nanyuan Formation) and/or coeval intrusions in Fujian Province. The sedimentary protoliths of these schists were likely deposited in fore-arc basins due to the diagnostic unimodal zircon age distribution patterns.(4) The metamorphic zircon overgrowths have relatively low Th/U ratios; they are dated at-130-100 Ma, with two peaks at-125 Ma and-108 Ma. The first metamorphic age of～125 Ma is slightly younger than the crystallized ages of gneiss, and also provides a minimum deposition age for sedimentary protoliths. In addition, this metamorphism is coincident with the timing of regional Early Cretaceous magmatic quiescence, likely corresponding to the late Early Cretaceous NW transpression of SE China. The second zircon metamorphic age of ～108 Ma is consistent with monazite ages of 108-101 Ma. The mineral assemblage garnet-plagioclase-biotite that contain monazites as inclusions yielded metamorphic conditions of ～600-650℃ and 3.3-5.2 kbar. The timing of this metamorphism and relatively high geothermal gradient correspond to the reinitiation of regional Cretaceous magmatism, which is likely related to a regional NW trending extension.(5) A geodynamic model of Early Cretaceous SE China is proposed to interpret these new results as well as the literature data. The subduction-related high-Al mafic source rocks with associated calc-alkaline granitoid rocks were formed in the Jurassic. The lithospheric extension and formation of Basin and Range-like province in the Jurassic time that is more or less similar to the rift setting facilitated the deep circulation of surface water interacting with existed metabasic intrusive rocks at high temperature and made them depleted in 18O. The Early Cretaceous (145-130 Ma) geology of Southeast China is characterized by intensive and widespread magmatism. The sedimentary protoliths of these schists were likely deposited in fore-arc basins in early Early Cretaceous (～141-130 Ma). Collision between the West Philippines and SE China blocks at-130-120 Ma resulted in the regional "magmatic quiescence" and formation of the Changle-Nan’ao Metamorphic Belt. At-116 Ma, reinitiation subduction of Paleo-Pacific Plate triggered the remelting of these 18O-depleted metabasic intrusive rocks in Early Cretaceous and generating the 18O-depleted magmas of the Xiaocuo granitoid complex; post-collisional extension accounted for the reinitiation of magmatism associated with amphibolite-facies metamorphism at～108 Ma.