The Luxi-Xianrenzhang diabase dikes were emplaced into the eastern part of the Guidong composite granitoids in northern Guangdong Province at the end of the Early Cretaceous. They show tholeiitic features, enrichment in large ion lithophile elements, slight enrichment in light rare earth elements, depletion in Zr and Hf, and basically no depletion in Nb and Ta and no Eu anomaly. They are similar to intraplate basalt in terms of trace element characteristics. They have high εNd(t) values (3.6–4.9), initial 87Sr/86Sr ratios (0.70530–0.70641) and δ18O values and Dupal anomaly of Pb isotope compositions. Their Sr-Nd, Pb-Sr, Pb-Nd and Pb-Pb isotopes plot between DMM and EMII, with Pb similar to EMII, Nd relatively close to DMM and Sr in between. This profile suggests that the diabase dikes studied were derived from partial melting of a mantle source that had been subjected to metasomatism by fluids originated from a subduction zone under a tectonic environment of crustal extension and lithosphere thinning in the late Yanshanian.
The Taohuadao, Qingtian and Laoshan granites are three typical late Yanshannian peralkaline granitic plu- tons in the coastal area, eastern China. In this paper, internal structures and chemical compositions of zircon from these A-type granites were investigated by electron microprobe. It is shown that zircon grains are mainly composed of two dis- tinctly separated parts. One is rich in Th (ThO2 >1 wt%, and ThO2/UO2 >2), and attains ThO2 up to 10.1 wt%; such value exceeds the dissolution limit of Th in the zircon structure (ThO2=5.5±2.5 wt%) determined in previous experiment. On the other hand, the other part is poor in Th (ThO2<1 wt%), but contains many thorite micro-inclusions with sieved tex- ture. By comparison, it is also implied that zircon in alumi- nous A-type granites is characterized by low content of ThO2 (<1 wt%), ThO2/UO2 <2 and absence of thorite inclusion. Based on mineralogical features, one is tempted to assume that the Th-rich zircon is formed during the early crystalli- zation of deep-sourced, high-temperature and Th-enriched A-type granitic magma. Such zircon is then subjected to late dissolution owing to accumulation of fluids at the end of magmatic evolution of A-type granite. Recrystallization fi- nally leads to formation of sieved low-Th zircon with thorite micro-inclusions, which may coexist with remnants of Th-rich zircons. The Th-rich zircon may be considered to be one of characteristic accessory minerals of peralkaline A-type granites.
High precision zircon U-Pb dating indicates that main intrusive bodies (Tong'an,Niumiao,Huashan,Lisong),and a mafic microgranular enclave in the Huashan-Guposhan complex were formed at 160-163 Ma.The εHf(t) values of zircons from the Huashan granite vary from -2.8 to +0.3 and those from the Lisong granite vary from -2.3 to +0.3,which are obviously different with those values (+2.6 to +7.4) of the mafic enclaves from the Lisong granite.These Hf isotopic data indicate that the mafic enclaves and host granites were crystallized from different sources of magmas,providing evidence for mafic-felsic magma mixing processes.The highest εHf(t) value of zircons from the mafic enclaves is up to +7.4,indicating that the mafic magma was originated from a relatively depleted mantle source.Studies on regional geology and the contemporaneous mafic and alkaline rocks in this area indicate that the mafic magma was not originated from reworking of basaltic juvenile crust,but from partial melting of the mantle.However,it remains to be resolved whether the mafic magma was derived from partial melting of the asthenosphere or the lithospheric mantle.The Huashan granite and the Lisong granite were formed from hybrid magma of mantle-derived and crust-derived magmas,and the mafic enclaves are considered as remains of mantle-derived magma during mixing processes.The εHf(t) values of zircons from the Niumiao diorite vary from -1.1 to +2.1,and those from the Tong'an quartz monzonite vary from -1.7 to +1.7.These values are lower than those from the mafic enclaves,suggesting that the diorite and monzonite were formed from different source-derived magmas with the mafic enclaves.The εHf(t) values for the Niumiao diorite and the Tong'an monzonite are only slightly higher than those for the Hushan granite and the Lisong granite.Abundant mafic enclaves also occur in the Niumiao diorite and in the Tong'an monzonite.Thus,we suggest that the Niumiao diorite and the Tong'an monzonite were probably also formed from the same hybrid magma as the granit
ZHAO KuiDong JIANG ShaoYong ZHU JinChu LI Liang DAI BaoZhang JIANG YaoHui LING HongFei
