Patrick Asamoah Sakyi

SIMS zircon U-Pb geochronology and Sr-Nd isotopes of Ni-Cu-Bearing Mafic-Ultramafic Intrusions in Eastern Tianshan and Beishan in correlation with flood basalts in Tarim Basin (NW China): Constraints on a ca. 280 Ma mantle PLUME

Zircon SIMS U-Pb dating of the Poshi, Hongshishan, Bijiashan, and Huangshan Ni-Cu-bearing and Xiangshan Ni-Cu-Ti-Fe-bearing mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift yields a relatively restricted range of 278.6 Ma to 284.0 Ma. The histogram of compiled age data of basalts in the Tarim Basin and mafic-ultramafic intrusions in the Eastern Tianshan and Beishan Rift has a peak of 280 Ma, which probably represents the time of mantle plume activity. The basalts have lower εNd(t) values in the range of −9.2 ∼ −1.7 and Mg# of <50, and higher TiO2 contents (>2 wt.%), indicating that they were generated directly from a peripheral zone of the mantle plume by low degree of melting. The mafic-ultramafic intrusions have higher εNd(t) of −1.3 ∼ 11.2 and Mg# of 33 ∼ 90, and lower TiO2 < 1.8 weight percent, suggesting that their parental magmas were produced from lithospheric mantle source by high degree of melting due to higher temperature of the mantle plume head. A possible mantle plume model beneath lithospheric mantle of the Tarim Basin, Tianshan and Beishan and its spatial framework is suggested.

Occurrence of an Alaskan-type complex in the Middle Tianshan Massif, Central Asian Orogenic Belt: inferences from petrological and mineralogical studies

The Xiadong mafic–ultramafic complex lies in the central part of the Middle Tianshan Massif (MTM), along the southern margin of the Central Asian Orogenic Belt (CAOB). This complex is composed of dunite, hornblende (Hbl) clinopyroxenite, hornblendite, and Hbl gabbro. These rocks are characterized by adcumulated textures and variable alteration. Orthopyroxene is an extremely rare mineral in all rock units and plagioclase is absent in dunite and Hbl clinopyroxenite. Hbl, Fe-chromite, and Cr-magnetite are common phases. Olivines have forsterite (Fo) contents ranging from 92.3 to 96.6. Clinopyroxenes are Ca-rich, Ti-poor diopsides, and mostly altered to tremolites or actinolites. Chromites display low TiO2 and Al2O3 contents and high Cr# and Fe2+/(Fe2+ + Mg) values. Primary and secondary Hbls show wide compositional variations. These petrological and mineralogical features as well as mineral chemistry are comparable to typical Alaskan-type complexes worldwide, which are widely considered to have formed above subduction zones. The chemistry of clinopyroxene and chromite supports an arc plate-tectonic origin for the Xiadong complex. Its confirmation as an Alaskan-type complex implies that the MTM, with Precambrian basement, was probably a continental arc during oceanic plate underflow and further supports the hypothesis of southward subduction of the Palaeozoic Junggar Ocean.

Identifying key processes in the hydrochemistry of a basin through the combined use of factor and regression models

An innovative technique of measuring the intensities of major sources of variation in the hydrochemistry of (ground) water in a basin has been developed. This technique, which is based on the combination of R-mode factor and multiple regression analyses, can be used to measure the degrees of influence of the major sources of variation in the hydrochemistry without measuring the concentrations of the entire set of physico-chemical parameters which are often used to characterize water systems. R-mode factor analysis was applied to the data of 13 physico-chemical parameters and 50 samples in order to determine the major sources of variation in the hydrochemistry of some aquifers in the western region of Ghana. In this study, three sources of variation in the hydrochemistry were distinguished: the dissolution of chlorides and sulfates of the major cations, carbonate mineral dissolution, and silicate mineral weathering. Two key parameters were identified with each of the processes and multiple regression models were developed for each process. These models were tested and found to predict these processes quite accurately, and can be applied anywhere within the terrain. This technique can be reliably applied in areas where logistical constraints limit water sampling for whole basin hydrochemical characterization. Q-mode hierarchical cluster analysis (HCA) applied to the data revealed three major groundwater associations distinguished on the basis of the major causes of variation in the hydrochemistry. The three groundwater types represent Na–HCO3, Ca–HCO3, and Na–Cl groundwater types. Silicate stability diagrams suggest that all these groundwater types are mainly stable in the kaolinite and montmorillonite fields suggesting moderately restricted flow conditions.

Geochronologic-petrochemical studies of the Hongshishan mafic-ultramafic intrusion, Beishan area, Xinjiang (NW China): Petrogenesis and tectonic implications

The Hongshishan mafic–ultramafic intrusion (SIMS zircon U–Pb age 286.4 ± 2.8 Ma) consists of dunite, clinopyroxene peridotite, troctolite, and gabbro. Major elements display systematic correlations. Trace elements have identical distribution patterns, including flat rare-earth element (REE) patterns with positive Eu anomalies and enrichments in large ion lithophile elements (LILE) but depletions in Nb and Ta, indicating fractional crystallization as a key factor in magmatic evolution. Petrologic and geochemical variations in drill core samples demonstrate that minor assimilation and progressive magma injections were closely associated with Ni–Cu mineralization. Mass balance estimates and Sr–Nd isotopes reveal that the Hongshishan parental magmas were high-Mg and low-Ti tholeiitic basalts and were derived from a lithospheric mantle source that had been modified by subducted slab metasomatism before partial melting. Southward subduction of the Palaeo-Tianshan–Junggar Ocean is further constrained by a compilation of inferred, subduction-induced modifications of mantle sources in mafic–ultramafic intrusions distributed in the eastern Tianshan–Beishan area. Integrating the regional positive ϵNd(t) granites, high-Mg and low-Ti basaltic magmas (mafic–ultramafic intrusions), and slightly later high-Ti basalts in NW China suggests that their petrogenesis could be attributed to Permian mantle plume activities.

Olivine Compositional Mapping of Mafic-Ultramafic Complexes in Eastern Xinjiang (NW China): Implications for Cu-Ni Mineralization and Tectonic Dynamics

Early Permian mafic-ultramafic complexes in eastern Xinjiang (新疆) are mainly distributed in the Beishan (北山) area, Mid-Tianshan (天山) massif and Jueluotage (觉罗塔格) belt. Systematic compositional mapping of olivines from these Early Permian mafic-ultramafic complexes demonstrates that an apparently spatial distribution and heterogeneous partial melting in the mantle source exists from the Beishan area, across the Mid-Tianshan massif, to the Jueluotage belt from the south to the north. This is probably consistent with the spatial evolutional differences and tectonic features of these three belts. The decreasing degree of partial melting, as revealed by decreasing Fo contents of olivines, from south to north and from east to west reflects the southward subduction of the Paleo-Asian Ocean and the south location of the indistinct mantle plume in the Permian. Simultaneously, NiO and Fo-mapping in olivine also indicates that sulfide segregation before olivine crystallization played an important role in Ni-Cu mineralization in the mafic-ultramafic complexes. Olivines with the compositional range of Fo (77–86) and NiO (less than 0.22 wt.%) are more favorable for Ni-Cu sulfide mineralization.

Late Paleozoic metallogenesis and evolution of the East Tianshan Orogenic Belt (NW China, Central Asia Orogenic Belt)

One of the most largest known and important metallogenic provinces in China is East Tianshan, where seven major types of Late Paleozoic metal deposits have been recognized: (1) porphyry-type Cu-Mo-(Au) ore deposits, (2) volcanic Fe-Cu deposits, (3) orogenic lode gold deposits, (4) magmatic Cu-Ni sulfide deposits, (5) epithermal gold deposits, (6) volcanic hydrothermal Cu deposits, and (7) skarn Cu-Ag deposits. Tectonically, the development of these Late Paleozoic metal mineral deposits was closely associated with the subduction and closure of the ancient Tianshan ocean intervening between the Tarim craton and the Junggar-Kazakhstan block. In the Late Devonian to Early Carboniferous, the northern margin of the Tarim craton existed as a passive-type continental margin, whereas the ancient Tianshan ocean was subducted beneath the southern margin of the Junggar-Kazakhstan block, resulting in the formation of the Dananhu-Tousuquan magmatic arc and associated porphyry-type Cu-Mo-(Au) deposits. In the Middle Carboniferous, the ancient Tianshan ocean began to subduct beneath the northern margin of the Tarim craton, leading to the formation of the Aqishan-Yamansu magmatic arc and associated volcanic Fe-Cu deposits. In the Late Carboniferous, the ancient Tianshan ocean was closed, and a continent-arc collision occurred, leading to the formation of the Tianshan orogen. Following the collision was an extensional event, which was associated with the emplacement of large amounts of ultramafic-mafic complexes and the formation of a number of large- to medium-scale magmatic copper-nickel ore deposits along the Kangger suture zone. In the Early Permian, East Tianshan entered into a post-collision stage, associated with the widespread emplacement of granitoid bodies and eruption of within-plate volcanism, which led to the formation of volcanic hydrothermal copper deposits, skarn-type Cu-Ag deposits, post-orogenic gold deposits, and epithermal gold deposits in East Tianshan.

Breakdown of orthopyroxene contributing to melt pockets in mantle peridotite xenoliths from the Western Qinling, central China: constraints from in situ LA-ICP-MS mineral analyses

Major and trace element compositions of constituent minerals, partly decomposed rims of orthopyroxenes (DRO), ‘closed’ melt pockets (CMP) and open melt pockets (OMP) in some Western Qinling peridotite xenoliths were obtained by LA-ICP-MS. Systematic core-to-rim compositional variations of garnet, clinopyroxene and orthopyroxene demonstrate that these minerals underwent variable degrees of subsolidus breakdown or partial melting. Both DROs and CMPs consist of similar mineral assemblages and are characterized by high TiO2, CaO + Na2O and low MgO contents; they are enriched in LREE and LILE compositions, have positive anomalies in Pb, Sr and particularly Ti, negative Th and U, and variable Zr and Hf anomalies. These chemical features are distinct and reflect reactions involving the orthopyroxenes. Compared to the CMPs, the OMPs, which are composed of a complex assemblage of minerals, display lower FeO and MgO contents, larger ranges in SiO2 and Na2O, higher TiO2, Al2O3, CaO and trace element concentrations, slightly negative Zr and Hf anomalies, and apparently negative Ti anomalies. Modeling calculations of partial fusion of orthopyroxenes and clinopyroxenes suggest that the CMPs most likely originated from the breakdown of orthopyroxenes with variably minor contribution of external melts from the melting of clinopyroxenes, whereas the OMPs were probably formed from the modification of the CMPs through the interaction with large amount of external melts.

A new modification of the sample introduction system for Os isotope ratio measurements

Here we present a new modification of the sample introduction system aimed at precisely measuring Os isotope ratios. A small glass connector was used to transfer OsO4 vapor from a Carius tube into a nebulizer. After dissolution of sample in a Carius tube, Os isotopes in the sample were determined simultaneously through in situdistillation without any prior separation procedure. The analytical precision at intermediate precision conditions of measurement was improved through the simultaneous introduction of Os via sparging and the aspiration of a solution containing Ir for mass bias correction. The present method, which involves the passage of Os vapor directly into the ICP-MS torch, thus solves the problems encountered previously when optimizing an instrument. The dating results of reference materials (RMs) HLP and JDC by the modified method are 221.8 ± 0.70 Ma (0.31%, 2s, n = 7) and 140.0 ± 0.44 Ma (0.31%, 2s, n = 9) respectively, which are consistent with their certified values of 221.4 ± 5.6 Ma and 139.6 ± 3.8 Ma respectively. Also reported in this study are dating results of several unknown molybdenite samples by the new method.

Large Lithium Isotopic Variations in Minerals from Peridotite Xenoliths from the Eastern North China Craton

To investigate the effects of melt-rock interaction on Li isotope fractionation, we report in situ Li concentrations and δ7Li of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx) for six peridotite xenoliths from the eastern North China Craton. These xenoliths contain two lherzolites, two Cpx-rich lherzolites, and two wehrlites and are variably metasomatized. Lithium isotope zonation is observed in most peridotite minerals. The majority of Cpx grains display isotopically light cores with lower Li concentrations than the heavy rims. However, the Opx grains show a different style of zonation from Cpx, where higher Li concentrations in the cores are associated with much lighter δ7Li. Olivines in most peridotites have a restricted range of Li concentration and δ7Li within individual grains, whereas the olivines in a lherzolite show isotopically light cores (−10.3) with high Li concentrations (2.3 ppm) and heavy rims (5.5) with low Li concentrations (1.7 ppm). These Li isotopic variations in mineral phases may reflect the combined effects of diffusion-driven kinetic fractionation of Li isotopes during melt/fluid-peridotite interactions and slow cooling. Intersample heterogeneity of Li isotopes is also apparent. Olivine with forsterite (Fo) content of 91.3 in one lherzolite sample has “normal” mantle-like Li concentrations (1.1∼2.4 ppm) and light δ7Li (−10.3∼5.5), while Ol with Fo content of 89.7 in another lherzolite has slightly high Li concentrations (2.0∼3.0 ppm) but similar δ7Li (1.6∼6.4) relative to normal mantle. Olivines in Cpx-rich lherzolites have lower Fo contents (83.8–87.5), higher Li concentrations (1.4∼4.5 ppm), and heavier δ7Li (5.0∼22.0) than those in lherzolites and normal mantle. The δ7LiOl value correlates positively with Li concentration and negatively with Fo from lherzolites to Cpx-rich lherzolites, indicating a reaction between lherzolites and melts with isotopically heavy Li- and Fe-rich signatures. By contrast, olivines in wehrlites have extremely lower Fo contents (82.2∼83.2) and higher Li concentrations (2.4∼4.2 ppm) than those in normal mantle, while their δ7Li values are within the range of normal mantle, reflecting metasomatism of the peridotites by asthenospheric melt. Overall, the large intragrain and intersample variations in Li concentrations and isotopic compositions reflect kinetic isotope fractionation during multiple metasomatisms.

Early Mesozoic tectono-magmatic activity and mineralization in Northeast China: evidence from Re–Os to U–Pb studies of the Taipingchuan porphyry Cu–Mo deposit in the Derbugan metallogenic belt

The Taipingchuan Cu–Mo deposit is a recently discovered large porphyry deposit located in the north of the Derbugan metallogenic belt of northeastern China. The geochronological data of the deposit yielded a Late Triassic zircon U–Pb age of 202 ± 6 Ma from a granodiorite porphyry that hosts the Cu–Mo mineralization. Measured Re–Os isotopes of seven disseminated molybdenite samples yielded an isochron age of 200 ± 5 Ma with mean square of weighted deviates of 2.7, while those of seven veinlet molybdenite samples also produced an isochron age of 200.1 ± 2.5 Ma and mean square of weighted deviates of 3.3. These isochron ages show that a Cu–Mo mineralization event occurred at ca. 200 Ma. Based on regional tectonic evolution, we propose that the Late Triassic Cu–Mo mineralization of the host porphyry in the Derbugan metallogenic belt was mainly associated with the subduction of the Mongol–Okhotsk Ocean slab under the Ergun block, contrary to previous suggestion that it was related to the subduction of the Mesozoic Palaeo-Pacific plate.