In this paper we report geochemical and Nd-Sr isotopic data for a late Archean gneissic granitic pluton （Hengling pluton）, an early Paleoproterozoic complex （Xipan complex） and a late Paleoproterozoic granitic plu...In this paper we report geochemical and Nd-Sr isotopic data for a late Archean gneissic granitic pluton （Hengling pluton）, an early Paleoproterozoic complex （Xipan complex） and a late Paleoproterozoic granitic pluton （Yunzhongshan granites） from the Liiliang-Wutai terrain, North China, to trace the source of these late Archean-Paleoproterozoic granitoids and, particularly, to understand the nature and mechanism of continental growth at that time. The Hengling granitic gneisses （ca. 2.51 Ga） are characterized by high Na2O and LILEs, TTG-like REE patterns （highly depleted HREE and minor Eu anomalies） and moderately depleted Nd-Sr isotopic compositions （εNd（t） =1.2-2.7, ISr=0.7015-0.7019）, and were considered as being products of arc magmatism that was developed upon the North China craton. The Xipan complex （ca. 2.2 Ga） contain gabbroic diorite and monzonite, mostly being Na2O-rich, highly fractionated REE patterns and isotopically enriched （εNd（t） =-1.5 to -4.1, Isr=0.7038-0.706）. The gabbroic diorites probably originated from melting of an enriched mantle source, but significantly contaminated by lower crustal material, and the monzonites probably represent a product of a mixture between the gabbroic dioritic magma and granitic melts of crustal origin. The Yunzhongshan post-collisional granitoids （ca. 1.8 Ga） are characterized by high-K affinity and highly-enriched and homogeneous Nd isotopic compositions （εNd（t）=-4.9 to -5.7）, although they split into two groups in terms of REE patterns： one group showing elevated HREE （and Sc, Y and Zr） with significant negative Eu anomalies and the other showing highly depleted HREE and, to a lesser extent, mid-REE with negligible Eu anomalies. These granites are genetically related to a process of extensional collapse of a thickened orogen. They formed through magma mixing between mantle-derived basaltic magmas and crust-derived granitic melts, followed by significant fractionation of ferromagnesian phases （like hornblende and Cpx） and feldspar and accessory zircons. Some Yunzhongshan granites show very old Nd model ages （2.9-3.0 Ga）, suggesting the existence of continental crust older than 2.7 Ga, which is supported by our zircon Hf isotopic data for these granites.展开更多
Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os con- tents, which makes it the most resistant phase remain...Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os con- tents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sec- tions of ophiolites. The podiform chromite is one of the ro- bust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic mélange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the 187Os/188Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (>300 ng/g), and their 187Os/188Os ratios range from 0.11021 to 0.11030, averaging 0.11026 ± 0.00005 (σ ), equivalent to a γOs = ?0.12 ± 0.06 at 2.6 Ga, which means that the Os isotopic composition of con- vecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.展开更多
As one of the areas where typical late Archean crust is exposed in the Eastern Block of the North China Craton, the northern Laioning Complex consists principally of tonalitic-trondhjemitic-granodioritic （TTG） gneis...As one of the areas where typical late Archean crust is exposed in the Eastern Block of the North China Craton, the northern Laioning Complex consists principally of tonalitic-trondhjemitic-granodioritic （TTG） gneisses, massive granitoids and supracrustal rocks. The supracrustal rocks, named the Qingyuan Group, consist of interbedded amphibolite, hornblende granulite, biotite granulite and BIF. Petrological evidence indicates that the amphibolites experienced the early prograde （M1）, peak （M2） and post-peak （M3） metamorphism. The early prograde assemblage （M1） is preserved as mineral inclusions, represented by actinotite ＋ hornblende - plagioclase ＋ epidote ＋ quartz 4- sphene, within garnet porphyroblasts. The peak assemblage （M2） is indicated by garnet ＋ clinopyroxene ＋ hornblende ＋ plagioclase ＋ quartz ＋ ilmenite, which occur as major mineral phases in the rock. The post-peak assemblage （M3） is characterized by the garnet 4- quartz symplectite. The P-T pseudosections in the NCFMASHTO system constructed by using THERMOCALC define the P-T conditions of M1, M2 and M3 at 490-550 C＋（4.5 kbar, 780 810 C/7.65- 8.40 kbar and 630-670 ＋C]8.15-9.40 kbar, respectively. As a result, an anticlockwise P-T path involving isobaric cooling is inferred for the metamorphic evolution of the amphibolites. Such a P-T path suggests that the late Archean metamorphism of the northern Liaoning Complex was related to the intrusion and underplating of mantle-derived magmas. The underplating of voluminous mantle-derived magmas leading to metamorphism with an anticlockwise P-T path involving isobaric cooling may have occurred in continental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift envi- ronments. A mantle plume model is favored because this model can reasonably interpret many other geological features of late Archean basement rocks from the northern Liaoning Complex in the Eastern Block of the North China Craton as well as their anticlockwise P-T paths involving isobaric cooling.展开更多
基金This study is financially supported by the National Natural Science Foundation of China （No. 40420120135）.
文摘In this paper we report geochemical and Nd-Sr isotopic data for a late Archean gneissic granitic pluton （Hengling pluton）, an early Paleoproterozoic complex （Xipan complex） and a late Paleoproterozoic granitic pluton （Yunzhongshan granites） from the Liiliang-Wutai terrain, North China, to trace the source of these late Archean-Paleoproterozoic granitoids and, particularly, to understand the nature and mechanism of continental growth at that time. The Hengling granitic gneisses （ca. 2.51 Ga） are characterized by high Na2O and LILEs, TTG-like REE patterns （highly depleted HREE and minor Eu anomalies） and moderately depleted Nd-Sr isotopic compositions （εNd（t） =1.2-2.7, ISr=0.7015-0.7019）, and were considered as being products of arc magmatism that was developed upon the North China craton. The Xipan complex （ca. 2.2 Ga） contain gabbroic diorite and monzonite, mostly being Na2O-rich, highly fractionated REE patterns and isotopically enriched （εNd（t） =-1.5 to -4.1, Isr=0.7038-0.706）. The gabbroic diorites probably originated from melting of an enriched mantle source, but significantly contaminated by lower crustal material, and the monzonites probably represent a product of a mixture between the gabbroic dioritic magma and granitic melts of crustal origin. The Yunzhongshan post-collisional granitoids （ca. 1.8 Ga） are characterized by high-K affinity and highly-enriched and homogeneous Nd isotopic compositions （εNd（t）=-4.9 to -5.7）, although they split into two groups in terms of REE patterns： one group showing elevated HREE （and Sc, Y and Zr） with significant negative Eu anomalies and the other showing highly depleted HREE and, to a lesser extent, mid-REE with negligible Eu anomalies. These granites are genetically related to a process of extensional collapse of a thickened orogen. They formed through magma mixing between mantle-derived basaltic magmas and crust-derived granitic melts, followed by significant fractionation of ferromagnesian phases （like hornblende and Cpx） and feldspar and accessory zircons. Some Yunzhongshan granites show very old Nd model ages （2.9-3.0 Ga）, suggesting the existence of continental crust older than 2.7 Ga, which is supported by our zircon Hf isotopic data for these granites.
文摘Podiform chromite deposits are a characteristic feature of the mantle sequences of harzburgitic ophiolites. The chromites usually have very low Re and high Os con- tents, which makes it the most resistant phase remaining from the primary magmatic history of the ultramafic sec- tions of ophiolites. The podiform chromite is one of the ro- bust indicators of initial Os isotopic compositions of the ophiolites where podiform chromites were derived from, which provides strong evidence for the origin and evolution of oceanic lithosphere. The Re and Os contents and the Os isotopic compositions of seven podiform chromitites from Zunhua ophiolitic mélange belt, North China are reported in this study. The Re contents range from 0.019 to 0.128 ng/g, Os from 8.828 to 354.0 ng/g, and the 187Os/188Os ratio from 0.11003 to 0.11145. Three massive chromitites among the sample set have very high Os contents (>300 ng/g), and their 187Os/188Os ratios range from 0.11021 to 0.11030, averaging 0.11026 ± 0.00005 (σ ), equivalent to a γOs = ?0.12 ± 0.06 at 2.6 Ga, which means that the Os isotopic composition of con- vecting upper mantle is chondritic in late Archean. It is the Os isotopic composition of podiform chromitites that are derived from the oldest ophiolite in the world till now.
基金financially funded by Chinese NSFC Grants(41190075,40730315, 40872123 and 41072152)Hong Kong RGC GRF grants(7066/ 07P and 7053/08P)
文摘As one of the areas where typical late Archean crust is exposed in the Eastern Block of the North China Craton, the northern Laioning Complex consists principally of tonalitic-trondhjemitic-granodioritic （TTG） gneisses, massive granitoids and supracrustal rocks. The supracrustal rocks, named the Qingyuan Group, consist of interbedded amphibolite, hornblende granulite, biotite granulite and BIF. Petrological evidence indicates that the amphibolites experienced the early prograde （M1）, peak （M2） and post-peak （M3） metamorphism. The early prograde assemblage （M1） is preserved as mineral inclusions, represented by actinotite ＋ hornblende - plagioclase ＋ epidote ＋ quartz 4- sphene, within garnet porphyroblasts. The peak assemblage （M2） is indicated by garnet ＋ clinopyroxene ＋ hornblende ＋ plagioclase ＋ quartz ＋ ilmenite, which occur as major mineral phases in the rock. The post-peak assemblage （M3） is characterized by the garnet 4- quartz symplectite. The P-T pseudosections in the NCFMASHTO system constructed by using THERMOCALC define the P-T conditions of M1, M2 and M3 at 490-550 C＋（4.5 kbar, 780 810 C/7.65- 8.40 kbar and 630-670 ＋C]8.15-9.40 kbar, respectively. As a result, an anticlockwise P-T path involving isobaric cooling is inferred for the metamorphic evolution of the amphibolites. Such a P-T path suggests that the late Archean metamorphism of the northern Liaoning Complex was related to the intrusion and underplating of mantle-derived magmas. The underplating of voluminous mantle-derived magmas leading to metamorphism with an anticlockwise P-T path involving isobaric cooling may have occurred in continental magmatic arc regions, above hot spots driven by mantle plumes, or in continental rift envi- ronments. A mantle plume model is favored because this model can reasonably interpret many other geological features of late Archean basement rocks from the northern Liaoning Complex in the Eastern Block of the North China Craton as well as their anticlockwise P-T paths involving isobaric cooling.
