Li Qiugen

EPMA Th-U-Pb Monazite Dating of Zhongtiao and Lüliang Precambrian Metamorphic Complexes

Abstract Zhongtiao and Luliang Precambrian metamorphic complexes are essential parts of the southern segment of Trans-North China Orogen (TNCO), the North China Craton (NCC). Their metamorphism and related granitic magmatism enable to better understand the nature and the evolution of the TNCO, the NCC, with great scientific significance. EPMA Th-U-Pb monazite dating of two monzogranitic rock samples from Yongji and Henglingguan intrusions in the Zhongtiao Precambrian complex yields two peak ages of 1884.7 Ma-1849.9 Ma, and 1743.5–1738.8 Ma, respectively. The former is interpreted as the main metamorphic ages and the latter as the ages of fluid activation. Five samples from the Luliang Precambrian complex were dated by EPMA Th-U-Pb monazite dating method. The garnet two-mica schist and the granitoid gneiss recorded three age peaks or range of >1902 Ma, 1883.3 Ma-1865.6 Ma, and 1731.3 Ma, corresponding to early magmatic and anatectic events, peak metamorphism, and late fluid activation, respectively. The two samples of garnet granite yield the ages of 1882.8 Ma-1850.9 Ma, corresponding to the anatectic granitic magmatism, which simultaneously occurred with the peak metamorphism. The unmetamorphosed granite vein yields the age of 1742.6 Ma, which is interpreted as the forming age of the granite vein and together with the age of 1731.3 Ma shown by the garnet two-mica schist may imply important fluid-magmatic activation events. The above dating results reveal that the peak metamorphism occurred during 1885 Ma-1849 Ma in the Zhongtiao and the Luliang Precambrian metamorphic complexes, accompanied by the S-type granitic magmatism. The age range of the peak metamorphism and cogenetic S-type granitic magmatism in the Zhongtiao and Luliang Precambrian complexes are consistent with the metamorphic and the deformation events in the Hengshan–Wutai–Fuping complexes, further proving that the amalgamation between the Western and the Eastern continental blocks in the NCC occurred during late Paleoproterozoic.

Zircon U-Pb and geochemical analyses for leucocratic intrusive rocks in pillow lavas in the Danfeng Group, north Qinling Mountains, China

Field observation showed that there are many irregular leucocratic intrusive rocks in pillow lavas in the Danfeng Group in the Xiaowangjian area, north Qinling orogenic belt. Photomicrographs indicated that the protoliths of those altered leucocratic intrusive rocks are dioritic rocks. Geochemical analyses showed that pillow lavas have a range of SiO2 from 47.35% to 51.20%, low abundance of TiO2 from 0.97% to 1.72%, and percentages of MgO (MgO#=41–49). Chondrite-normalized REE patterns of pillow lavas are even, indicative of a weak differentiation between LREE and HREE (La/YbN=1.52–0.99). N-MORB-normalized trace element abundances showed that pillow lavas are enriched in incompatible elements (e.g., K, Rb, and Ba). Leucocratic intrusive rocks in pillow lavas have a wide range of SiO2 from 53.85%–67.20%, low abundances of TiO2 from 0.51%–1.10%, and MgO (MgO#=40–51), and higher percentages of Al2O3 (13.32%–16.62%) and concentration of Sr (342–539 μg/g), ratios of Na2O/K2O (2–7) and Sr/Y (17–28). Chondrite-normalized REE patterns of leucocratic intrusive rocks showed highly differentiation between LREE and HREE (La/YbN=12.26–19.41). N-MORB-normalized trace element abundances showed that leucocratic intrusive rocks are enriched in incompatible elements (e.g., K, Rb, and Ba), and significantly depleted in HFSE (e.g., Nb, Ta, Zr and Ti), indicative of a relationship to subduction. Isotopically, leucocratic intrusive rocks have a similar εNd(t) (+7.45–+13.14) to that of MORB (+8.8–+9.7), which indicates that those leucocratic intrusive rocks sourced from depleted mantle most likely. SHRIMP U-Pb analyses for zircon showed that those leucocratic intrusive rocks were formed at 442±7 Ma, yielding an age of subduction in the early Paleozoic in the north Qinling orogenic belt.

Zircon SHRIMP geochronology and geochemistry of TTG rocks in Sushui Complex from Zhongtiao Mountains with its geological implications

Abstract The tonalite-trondhjermite-granodiorite (TTG) rocks in Sushui Complex of Zhongtiao Black can be divided into two series according to their zircon U-Pb ages and geochemical characteristics one is subduction-related (SR) and the other collision-related (CR). The SR TTG rocks, together with other Late Archean island arc magmatism, were developed as a result of oceanic subduction between the Eastern and Western black in Late Archean; while the CR TTG rocks formed in a thickened crustal environment. which was responding to the collision between the Eastern and Western blocks in Paleoproterozoic. All these features support a model that the Zhongtiao Block is a part of the Trans-North China Orogen in the middle of North China Craton. * Supported by the National Natural Science Foundation of China (Grant No. 40420120135)

Geochemistry and Zircon U–Pb–Hf Isotopic Systematics of the Sanchahe Quartz Monzonite Intrusion in the North Qinling Tectonic Zone, Central China: Implications for its Petrogenesis and Tectonic Setting

The Sanchahe quartz monzonite intrusion is situated in the middle segment of the North Qinling tectonic belt, Central China mainland, and consists chiefly of sanukitoid–like and granodioritic-monzogranitic rocks. The sanukitoid–like rocks are characterized by quartz monzonites, which display higher Mg# (55.0–59.0), and enrichments in Na2O+K2O (7.28–8.94%), Ni (21–2312 ppm), Cr (56–4167 ppm), Sr (553–923 ppm), Ba (912–1355 ppm) and LREE ((La/Yb)N=9.47–15.3), from negative to slightly positive Eu anomalies (δEu=+0.61 to +1.10), but also depletion in Nb, Ta and Ti. The granodioritic-monzogranitic rocks diaplay various Mg# of 6.00–53.0, high Na2O+K2O (7.20–8.30%), Sr (455–1081 ppm) and (La/Yb)N (27.6–47.8), with positive Eu anomalies (δEu=1.03–1.57) and depleted Nb, Ta and TL Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) zircon U-Pb isotopic dating reveals that the sanukitoid-like rocks were emplaced at two episodes of magmatism at 457±3 Ma and 431±2 Ma, respectively. The monzogranites were emplaced at 445±7Ma. Sanukitoid-like rocks have their ∊Hf(r) values ranging from +0.3 to +15.1 with Hf–depleted mantle model ages of 445 to 1056 Ma, and the monzogranite shows its ∊Hf(t) values ranging from -21.6 to +10.8 with Hf–depleted mantle model ages of 635 to 3183 Ma. Petrological, geochemical and zircon Lu–Hf isotopic features indicate that the magmatic precursor of sanukitoid–like rocks was derived from partial melting of the depleted mantle wedge materials that were metasomatized by fluids and melts related to subduction of oceanic slab, subsequently the sanukitoid magma ascended to crust level. This emplaced mantle magma caused partial melting of crustally metamorphosed sedimentary rocks, and mixing with the crustal magma, and suffered fractional crystallization, which lead to formations of quartz monzonites. However, the magmatic precursor of the granodioritic-monzogranitic rocks were derived from partial melting of subducted oceanic slab basalts. Integrated previous investigation for the adackitic rocks in the south of the intrusion, the Sanchahe intrusion signed that the North Qinling tectonic zone was developed in an early Paleozoic transitionally tectonic background from an island arc to back–arc.

Geochemical characteristics of the metapelites from the Xingxingxia group in the Eastern Segment of the Central Tianshan: Implications for the provenance and paleoweathering

Metapelites from the Mesoproterozoic Xingxingxia group in the Eastern Segment of the Central Tianshan, Northwestern China, were analyzed for major and trace elements, including rare earth elements. Compared with post-Archean shales, the metapelites are enriched in Sr, Hf and Zr. The other elements are similar to the compositions of the Phanerozoic North American Shale Composite and the Post-Archean Australian average Shale (PAAS). The characteristics of Al2O3/TiO2, Cr/Zr, Cr/Th, Th/Sc and high Zr concentration suggest that these pelites could be derived from a significant proportion of felsic and a few proportion of participation of mafic materials in nature. These rocks display highly fractionated REE patterns, and average (La/Yb)n = 18.6. Several samples have slightly negative Eu anomalies (Eu/Eu* = 0.7–0.84) in comparison with PAAS (Eu/Eu* = 0.65), indicating contribution from young immature materials. The low K2O/Al2O3 values of these metapelites suggest that their source materials could contain minimal alkali feldspar. In the A-CN-K triangular diagram, the sample data array intersects the feldspar join, indicating that the source had a plagioclase: K-feldspar ratio of approximately 5:1, representing weathered products from components that could approximately correspond to granodiorites and tonalites. The low Chemical Index of Alteration (CIA) and high Index of Compositional Variability (ICV) values suggest low degrees of weathering of the source and low compositional maturity of the sediments, respectively. The geochemical characteristics of the metapelites from the Xingxingxia group also demonstrate that these sediments were deposited in a back-arc background within a continental-arc system.

Petrogenesis of Paleoproterozoic Luyashan charnockitic rocks in Shanxi Province: Constraints from Geochemistry and Nd isotope

Abstract Luyashan charnockite pluton mainly consists of monzonite, adamellite, charnockite and syenogranite, which are characterized by the enrich of TiO2, P2O5, K2O, Zr, Nb, Y, Pb, La, Ce, Ba and a higher K2O/Na2O and depletion of MgO, CaO, Mg#, Th, U and lower Sr Ba and Rb Ba. The negative correlations between Zr, Nb, Ce and SiO2 are distinct from I-type granites. Isotopically Luyashan charnockite plutons are relatively uniform in Nd isotope, displaying initialϵNd (t) (−5.93 to −6.97) and Nd depleted mantle model ages (2.67 Ga to 2.78 Ga). These features indicate that Luyashan charnockitic magma derived from pre-existing late Archean crustal sources and the partial melting of mafic granulites probably under exceptionally high temperature with CO2-rich fluid. The garnet is a main residual phase during the partial melting. The original dry charnockitic magma experienced crystal fractionation of pyroxene, plagioclase, apatite, and ilmenite during early crystallization. The geochemical evidence suggests tha...