The Experts below are selected from a list of 2316 Experts worldwide ranked by ideXlab platform
Xun Wei - One of the best experts on this subject based on the ideXlab platform.
-
Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi Wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
International Geology Review, 2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi Wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi Wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi Wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5-48.7wt.%) and Na2O+K2O (2.4-3.8wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9-4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5-23.0wt.%) and very low Na2O (0.22-0.34wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7kbar) and T (1000 degrees C), and assumed H2O (~1.5wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the Wehrlite. The results confirm that H2O exerts a major control. Zircon delta O-18 (VSMOW) values (2.99-3.71 parts per thousand) are significantly lower than that of mantle-derived zircon (5.3 +/- 0.6 parts per thousand). Such low zircon delta O-18 values may be due to incorporation of large amounts of low-delta O-18, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
-
origin of high an plagioclase in the early permian 280 ma xiaohaizi Wehrlite northwest china insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
International Geology Review, 2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi Wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi Wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi Wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5-48.7wt.%) and Na2O+K2O (2.4-3.8wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9-4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5-23.0wt.%) and very low Na2O (0.22-0.34wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7kbar) and T (1000 degrees C), and assumed H2O (~1.5wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the Wehrlite. The results confirm that H2O exerts a major control. Zircon delta O-18 (VSMOW) values (2.99-3.71 parts per thousand) are significantly lower than that of mantle-derived zircon (5.3 +/- 0.6 parts per thousand). Such low zircon delta O-18 values may be due to incorporation of large amounts of low-delta O-18, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
-
Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi Wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi Wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi Wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi Wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the Wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
-
Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi Wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate Wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the Wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
-
Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
Journal of Petrology, 2014Co-Authors: Xun Wei, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi Wehrlite intrusion is composed mainly of olivine (Fo69-75), clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the Wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi Wehrlite intrusion and dykes. The Xiaohaizi Wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi Wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
Zhen-yu Luo - One of the best experts on this subject based on the ideXlab platform.
-
origin of high an plagioclase in the early permian 280 ma xiaohaizi Wehrlite northwest china insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
International Geology Review, 2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi Wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi Wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi Wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5-48.7wt.%) and Na2O+K2O (2.4-3.8wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9-4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5-23.0wt.%) and very low Na2O (0.22-0.34wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7kbar) and T (1000 degrees C), and assumed H2O (~1.5wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the Wehrlite. The results confirm that H2O exerts a major control. Zircon delta O-18 (VSMOW) values (2.99-3.71 parts per thousand) are significantly lower than that of mantle-derived zircon (5.3 +/- 0.6 parts per thousand). Such low zircon delta O-18 values may be due to incorporation of large amounts of low-delta O-18, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
-
Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi Wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
International Geology Review, 2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi Wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi Wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi Wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5-48.7wt.%) and Na2O+K2O (2.4-3.8wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9-4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5-23.0wt.%) and very low Na2O (0.22-0.34wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7kbar) and T (1000 degrees C), and assumed H2O (~1.5wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the Wehrlite. The results confirm that H2O exerts a major control. Zircon delta O-18 (VSMOW) values (2.99-3.71 parts per thousand) are significantly lower than that of mantle-derived zircon (5.3 +/- 0.6 parts per thousand). Such low zircon delta O-18 values may be due to incorporation of large amounts of low-delta O-18, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
-
Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi Wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes
2016Co-Authors: Xun Wei, Zhong-yuan Ren, Zhen-yu LuoAbstract:The Xiaohaizi Wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi Wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi Wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the Wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.
-
Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi Wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate Wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the Wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
Attila Novák - One of the best experts on this subject based on the ideXlab platform.
-
Metasomatism-induced Wehrlite formation in the upper mantle beneath the Nógrád-Gömör Volcanic Field (Northern Pannonian Basin): Evidence from xenoliths
Geoscience Frontiers, 2020Co-Authors: Levente Patkó, Nóra Liptai, László Előd Aradi, Rita Klébesz, Eszter Sendula, Robert J. Bodnar, István János Kovács, Károly Hidas, Bernardo Cesare, Attila NovákAbstract:Abstract Clinopyroxene-enriched upper mantle xenoliths classified as Wehrlites are common (~20% of all xenoliths) in the central part of the Nograd-Gomor Volcanic Field (NGVF), situated in the northern margin of the Pannonian Basin in northern Hungary and southern Slovakia. In this study, we thoroughly investigated 12 Wehrlite xenoliths, two from each Wehrlite-bearing occurrence, to determine the conditions of their formation. Specific textural features, including clinopyroxene-rich patches in an olivine-rich lithology, orthopyroxene remnants in the cores of newly-formed clinopyroxenes and vermicular spinel forms all suggest that Wehrlites were formed as a result of intensive interaction between a metasomatic agent and the peridotite wall rock. Based on the major and trace element geochemistry of the rock-forming minerals, significant enrichment in basaltic (Fe, Mn, Ti) and high field strength elements (Nb, Ta, Hf, Zr) was observed, compared to compositions of common lherzolite xenoliths. The presence of orthopyroxene remnants and geochemical trends in rock-forming minerals suggest that the metasomatic process ceased before complete wehrlitization was achieved. The composition of the metasomatic agent is interpreted to be a mafic silicate melt, which was further confirmed by numerical modelling of trace elements using the plate model. The model results also show that the melt/rock ratio played a key role in the degree of petrographic and geochemical transformation. The lack of equilibrium and the conclusions drawn by using variable lherzolitic precursors in the model both suggest that wehrlitization was the last event that occurred shortly before xenolith entrainment in the host mafic melt. We suggest that the wehrlitization and the Plio–Pleistocene basaltic volcanism are related to the same magmatic event.
A L Perchuk - One of the best experts on this subject based on the ideXlab platform.
-
an ordovician age of the dunite Wehrlite clinopyroxenite banded complex of the nurali massif southern urals russia u pb shrimp zircon age data
Moscow University Geology Bulletin, 2018Co-Authors: A A Krasnobaev, P M Valizer, A L PerchukAbstract:The data on the geochemistry and geochronology of zircons from Wehrlites and clinopyroxenites of the dunite–Wehrlite–clinopyroxenite banded complex that lies at the base of the crustal section of the ophiolite complex of the Nurali massif are presented. The obtained U–Pb age of the banded complex of 450 ± 4 Ma differs markedly from the previous age data. According to REE distribution patterns zircons from ultramafic rocks are attributed to the magmatic type and they indicate the age and supposed genetic similarity of the above rocks with lherzolites and dunites from the mantle section of the Nurali massif.
-
An Ordovician Age of the Dunite–Wehrlite–Clinopyroxenite Banded Complex of the Nurali Massif (Southern Urals, Russia): U–Pb (SHRIMP) Zircon Age Data
Moscow University Geology Bulletin, 2018Co-Authors: A A Krasnobaev, P M Valizer, A L PerchukAbstract:The data on the geochemistry and geochronology of zircons from Wehrlites and clinopyroxenites of the dunite–Wehrlite–clinopyroxenite banded complex that lies at the base of the crustal section of the ophiolite complex of the Nurali massif are presented. The obtained U–Pb age of the banded complex of 450 ± 4 Ma differs markedly from the previous age data. According to REE distribution patterns zircons from ultramafic rocks are attributed to the magmatic type and they indicate the age and supposed genetic similarity of the above rocks with lherzolites and dunites from the mantle section of the Nurali massif.
Yuexing Feng - One of the best experts on this subject based on the ideXlab platform.
-
Composition of the Tarim mantle plume: Constraints from clinopyroxene antecrysts in the early Permian Xiaohaizi dykes, NW China
Lithos, 2015Co-Authors: Xun Wei, Jianxin Zhao, Zhen-yu Luo, Yuexing FengAbstract:Numerous alkaline basaltic dykes crosscut the Early Permian Xiaohaizi Wehrlite in drill-cores and syenite intrusion in the Tarim large igneous province, NW China. One basaltic dyke contains abundant clinopyroxene macrocrysts with strong resorption textures. Such a textural disequilibrium is consistent with their contrasting chemistry between the macrocrysts (Mg# = 80-89) and the host dyke (Mg# = 39, corresponding to Mg# = 73 of clinopyroxene in equilibrium with the dyke), indicating that they are not phenocrysts. The clinopyroxene macrocrysts are characterized by low TiO2 (026-1.09 wt.%), Al2O3 (1.15-3.10 wt.%) and Na2O (0.16-0.37 wt.%), unlike those in mantle peridotites but resembling those in layered mafic intrusions in the same area. The clinopyroxene macrocrysts and the clinopyroxenes from the Xiaohaizi cumulate Wehrlites define a coherent compositional trend and have identical trace element patterns, pointing to a comagmatic origin for these crystals. Accordingly, the macrocrysts cannot be xenoaysts foreign to the magmatic system. Rather they are antecrysts that crystallized from progenitor magmas and have been reincorporated into the host dyke before intrusion. The Sr-87/Sr-86(i) (0.7035-0.7037) and epsilon Nd-i (4.5-4.8) of the clinopyroxene macrocrysts with high Mg# (80-89) are apparently lower and higher than their respective ratios of the clinopyroxenes in the Wehrlites (Mg# = 75-84, Sr-87/Sr-86(i) = 0.7038-0.7041, epsilon Nd-i = 1.0-1.9). This difference in isotopes can be accounted for by assimilation and fractional crystallization (AFC) process operated during the formation of the Xiaohaizi intrusion. In this sense, the clinopyroxene macrocrysts record the composition of the uncontaminated Tarim plume-derived melts. (C) 2015 Elsevier B.V. All rights reserved.
-
Petrology and Sr-Nd Isotopic Disequilibrium of the Xiaohaizi Intrusion, NW China: Genesis of Layered Intrusions in the Tarim Large Igneous Province
Journal of Petrology, 2014Co-Authors: Xun Wei, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi Wehrlite intrusion is composed mainly of olivine (Fo69-75), clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the Wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi Wehrlite intrusion and dykes. The Xiaohaizi Wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi Wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
-
petrology and sr nd isotopic disequilibrium of the xiaohaizi intrusion nw china genesis of layered intrusions in the tarim large igneous province
Journal of Petrology, 2014Co-Authors: Yigang Xu, Chuanlin Zhang, Jianxin Zhao, Yuexing FengAbstract:Layered mafic intrusions (LMI) are sporadically distributed in the Early Permian Tarim large igneous province (LIP), NW China, and are crosscut by numerous contemporaneous dykes. The Xiaohaizi Wehrlite intrusion is composed mainly of olivine (Fo69-75), clinopyroxene (Mg# = 75-84), intercumulus plagioclase (An53-86) and Fe-Ti oxides. Both petrography and mineral compositions suggest that olivine and clinopyroxene crystallized earlier than plagioclase and Fe-Ti oxides. The dykes are of alkali basalt to trachyandesite with low Mg# (35-39). The least-contaminated dykes display strong rare earth element (REE) fractionation, enrichment of Nb and Ta, and depletion of Pb relative to other similarly incompatible elements, bearing strong similarity to ocean island basalts (OIB). This, together with their positive epsilon Nd-i values (4.3-4.8), is consistent with derivation from an enriched asthenospheric mantle source. Clinopyroxenes in the Wehrlites display convex-upward chondrite-normalized REE patterns. The melts in equilibrium with these clinopyroxenes have very similar trace element compositions to those of the crosscutting dykes, suggesting a similar mantle source shared by the Xiaohaizi Wehrlite intrusion and dykes. The Xiaohaizi Wehrlite intrusion is characterized by Sr-Nd isotopic disequilibrium between clinopyroxene and plagioclase separates: Sr-87/(86)Sri (0.7038-0.7041) and epsilon Nd-i (1.0-1.9) of clinopyroxene are lower and higher than the respective ratios of intercumulus plagioclase (Sr-87/Sr-86(i) = 0.7042-0.7043, epsilon Nd-i = 0.4-1.0). The Sr-87/(86)Sri and epsilon Nd-i of clinopyroxene separates correlate positively and negatively with Zr/ Nb, respectively, implying variable degrees of crustal contamination during the formation of the Xiaohaizi Wehrlite intrusion. Sr-87/(86)Sri increases and epsilon Nd-i decreases with increasing Ca content of plagioclase, indicating that higher An plagioclases experienced higher degrees of contamination. This can be explained by assimilation of continental crust through a turbulent magma ascent (ATA) process. However, this ATA model fails to account for the positive correlation between the Mg# and epsilon Nd-i of clinopyroxene separates. The isotopic disequilibrium in the Xiaohaizi LMI is more probably generated during an assimilation and fractional crystallization process involving Archean-Neoproterozoic basement and carbonates as contaminants.
