Petrogenesis of Early Cretaceous riebeckite granophyres in southern Da Hinggan Mts.

This Aliwula area in paper reports lithologic features, K-Ar age and geochemical data of riebeckite granophyres from the southern Da Hinggan Mts., aiming to reveal the petrogenesis of riebeckite granophyres. K- Ar age of riebeckite granophyres is 126± 2 Ma, implying that the riebeckite granophyres formed in the Early Cretaceous. The granophyres are rich in riebeckites and with a lot of melt-fluid inclusion in its quartz pheno- crysts. The granophyres are characterized by extensive enrichment in Si02, FeO, and （Na20 ＋ K20） and de- pletion in MgO and CaO, strong negative Eu anomalies and strong positive Ce anomalies. Additionally, the rie- beckite granophyres not only have high total REE contents and display enrichment of HFSEs （for example Zr, Hf, Nb, Ta）, but also are strong in depletion of LILEs （ e. g. Ba, Sr） as well as high Ga/A1 ratios. Primitive mantle-normalized REE pattern significantly displays REE M-W tetrad effect. REEs fractionate evidently and highly enrich in LREE, but are uneven distribution in the rocks. Taken together, we conclude that the riebeck- ite granophyres are similar to typical A-type granite, which could be derived from stretching environments in the Early Cretaceous. The granophyres originated from residual melt which underwent highly differentiation process, and were formed in magmatic-hydrothemal transition stage at last.

Geochemistry and tectonic significance of Early Cretaceous granites in Highland 1248 of Daxing'anling

The Early Cretaceous granitic complex in Highland 1248 of Daxing'anling successively consists of intruded granodiorite,monzonite granite and syenite granite. Through test analysis on the major,trace and rare earth elements of the intrusive complex,this study focuses on the source characteristics and tectonic environment of the original magma of intrusive complex. The results show that the intrusive rocks in Highland 1248 are meta-aluminous-peraluminous rocks in calc-alkaline series with homologous characteristics in δEu negative anomalies; the complex is enriched in LILE( Rb,Cs and K) and depleted in HFSE( lanthanide,Sc,Y,U,Nb and Ta),displaying the geochemical characteristics of I-type granites in active continental-margin subduction zones. The complex also has the characteristics of granites after the collision of plate with rich aluminum and high potassium,but significantly depleted Nb,Ta,Ti,P and other elements,i. e. in the orogenic evolutionary stage. The zircon U-Pb SHRIMP isotopic apparent age of the complex in Highland 1248 is 140. 0--141. 0Ma,and the formation time is in Early Cretaceous.

Geochronology and significance of volcanic rocks in the Ning-Wu Basin of China

The Ning-Wu(Nanjing-Wuhu) Basin is one of the most important volcanic basins in the Middle-Lower Yangtze River Valley,eastern China.It consists of four volcanic units,i.e.,the Longwangshan,Dawangshan,Gushan,and Niangniangshan Formations.Their LA-ICP MS U-Pb zircon ages are 134.8±1.8,132.2±1.6,129.5±0.8,and 126.8±0.6 Ma,respectively.Results of this study indicate that all volcanic rocks in the Ning-Wu Basin were formed in the Early Cretaceous from 135 to 127 Ma,lasting 8-10 m.y.No Jurassic volcanic activities occurred in any of the volcanic basins of the Middle-Lower Yangtze River Valley,including the Ning-Wu Basin.These new chronological results provide significant evidence for further study of this region to improve our understanding of Mesozoic tectonic,magmatic,and metallogenic processes of eastern China.

Modification of the lithospheric mantle by melt derived from recycled continental crust evidenced by wehrlite xenoliths in Early Cretaceous high-Mg diorites from western Shandong, China

This paper reports petrographic, mineral chemical, olivine oxygen isotopic, and whole-rock geochemical data for wehrlite xenoliths from the Early Cretaceous Tietonggou high-Mg diorites in western Shandong Province, in the eastern part of the North China Craton (NCC), and describes the origin of these wehrlites and the processes that affected the deep lithospheric mantle in this area.Wehrlite xenoliths are rounded and vary in size between 3 cm × 4 cm 5 cm and 3 cm 2 cm 1 cm.Olivine within these xenoliths occurs as an isolated residual phase within clinopyroxene, has Fo contents between 89 and 91, and contains between 1414 and 3629 ppm Ni, similar to the values of olivine from peridotite xenoliths in the Cenozoic basalts of eastern China, but lower than the values of olivine from harzburgite xenoliths in the Early Cretaceous high-Mg diorites in western Shandong.In situ oxygen isotope analysis yielded 18 O values of olivine from (6.03±0.33)‰ to (6.82±0.35)‰, averaging (6.5±0.4)‰; this is higher than typical mantle-derived olivine ((5.2±0.3)‰).Compared with clinopyroxenes from peridotite xenoliths in the Late Cretaceous and Cenozoic basalts, clinopyroxenes in the wehrlites contain relatively low concentrations of Na 2 O, TiO 2 , and Al 2 O 3 , high concentrations of CaO, and higher Mg # (91.2-94.1) and Ti/Eu ratios (2082-2845), being similar in composition to clinopyroxenes within harzburgite xenoliths in the Early Cretaceous high-Mg diorites.Clinopyroxenes from wehrlite xenoliths are characterized by low total REE abundance, enrichment in light REEs, and depletion in high field strength elements such as Nb, Ta, Zr, and Hf.Moreover, the 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and 187 Os/ 188 Os (125 Ma) ratios of these wehrlites vary from 0.70596 to 0.70737, 0.512181 to 0.512416, and 0.12661 to 0.57650, respectively.These data suggest that these wehrlite xenoliths were formed by modification of the lithospheric mantle by melts derived from recycled continental crust.