- Email: [email protected]
Major Precambrian Events in Northwestern China
692
RODINIA, GONDWANA AND ASIA
Major Precambrian Events in Northwestern China Songnian Lu Tianjin Institute of Geology and Mineral Resources, CGS, China Precambrian rocks in China are mainly exposed in the North China Platform. They also occur at the margins of the Tarim Platform, the Qaidam Block and the South China Platform. Although several studies have been carried out on Precambrian geology of North China Platform, data on Northwestern China are sparse. Based on our recent investigations and new isotopic ages, the oldest granitic gneiss is identified at the Altyn tagh fault belt, and the Neoproterozoic thermo-tectonic events are very significant. The area preserves records for terminal stages of both Archean and Paleoproterozoic in south and northwest China, and marks an important link in the evolution from Rodinia to Gondwanaland supercontinents. This work introduces the major Precambrian events in Northwestern China. The first and foremost is the identification of oldest rocks from the Altyn tagh, which is located in the eastern margin of Tarim Platform. According to previous data, the Milan Group is composed of quartz-feldspar gneisses and belongs to the Neoarchean, with ca. 2500 Ma U-Pb isotopic age. Our recent study shows that the Milan Groupconsists mainly of TTG and granitic gneisses, with only minor supracrustal components. Single zircon U-Pb age of granitic gniess yields 3605 k 43 Ma. Ages of 2500 Ma, 1900 Ma and 1100 Ma were also measured from the same sample, although 3600 Ma age preserved by zircons is significant. The Archean rocks of Kuruktag and Dunhuang regions in the Tarim Platform comprise tonalite, trondhjemite and granodiorite gneisses. Amphibolites occur as enclaves within the gneisses. A U-Pb age of 2582 11Ma was reported from the granitic gneiss at Kuruktag. The tonalitic and trondhjemitic gneisses yield upper intercepts U-Pb age of 2670 12 Ma, and lower intercept age of 1017 k 3 Ma. The former should represent the crystallization age and the latter might reflect the age of Neoproterozoic event. 2.68 Ga Nd model age from the same sample, which is similar to the above U-Pb age, reflects an important crust-forming event in this area. Paleoproterozoic and Mesoproterozoic rocks are largely preserved in the northern margin of Tarim, southwestern segment of North China Platform, Central Qilian and North Kulun Mountain. U-Pb isotopic age from the mylonitised granites here indicates 1499 21 Ma. Another sample of granitic gneiss from the northern part of East Kulun Mountain yields similar age of 1624 & 22 Ma. The granitic gneisses intruded into the Paleoproterozoic Jinshikou Group, which reflect an important magmatic activity at the end of Paleoproterozoic. Neoproterozoic rocks are widespread in the study area represented mainly by vast Neoproterozoic granitic gneiss, with supracrustal rocks preserved as enclaves or relicts. The protolith of these gneisses include a variety of granites. Ages of 1020 +. 41 Ma and 803 7 Ma have been obtained from trondhjemite
+-
and granodiorite gneisses, respectively. An age of 744 & 28 Ma is also measured from a K-rich granitic gneiss sample, which may reflect the time of a late or post orogenic event. The Quanji Group as the first sedimentary cover rests unconformably upon the metamorphic Dakendaban Complex. 738 +. 28 Ma from single zircon U-Pb dating is obtained from volcanic rocks of this group. Based on a series of geochronological data, the unconformity between the Quanji Group and Dakendaban Complex is placed at ca. 750-800 Ma ago. Eclogites within the Neoproterozoic granitic gneiss belt yield Sm-Nd mineral and whole rock isochron age of 545 5 8 Ma. U-Pb dating of eclogites in Yuqiahe and Shaliuhe shows 499.6 -t 6.5 Ma. On basis of the above ages from different methods, a significant Pan-African tectonothermal event between 550 Ma-500 Ma can be identified in this terrain, linking with similar events in Gondwanaland. Eight zircon grains from the host gneisses of the Liuyuan eclogites define a discordia line with an upper intercept age of 880 F 31 Ma. T,, age is about 1500 Ma, which is much older than 880Ma of U-Pb age of the gneiss. An eclogite sample from Gupuqian area yields upper intercept age of 861 k 71 Ma and lower intercept age of 440 k 29 Ma, suggesting two important events. Neoproterozoic pegmatite swarms are also preserved in the Altyn tagh fault Belt. Zircons from the pegmatite yield a U-Pb age of 913 +. 26 Ma, which illustrates that the pegmatite swarms were emplaced during the Neoproterozoic magmatic activity. Recently discovered eclogites in the southwestern segment of Altyn tagh gave a U-Pb zircon age of 504 & 5 Ma. High-pressure metamorphosed pelitic rocks in the central part of the fault belt show 575 k 3 Ma from Ar-Ar dating of phengite. We identify some similarity between northern a n d Northwestern China in the 3600 Ma magmatism and thermotectonic events a t terminal stages of Archean a n d Paleoproterozoic. However, the extensive development of Neoproterozoic events is largely confined to Northwestern China. The salient findings from our study are summarised as follows. (1)3600 Ma zircons were identified in the Altyn tagh fault belt, which represents the third oldest isotopic age record from China. (2) TTG and K-rich granitic gneisses developed in the terminal stages of Archean and Paleoproterozoicrespectively in both north and northwest China. (3) The granite emplacement is concentrated at the early Neoproterozoic period in Northwestern China. These are developed in the southern part of the north margin of the Qaidam basin, which may represent an important tectonic boundary. (4) The isotopic ages of ca. 550-500 Ma recorded from eclogites and other high-pressure metamorphic rocks in the study area may correspond to the imprints of PanAfrican orogeny in this area, which needs detailed evaluation in future studies.
Gondwnnn Rescorch, V. 4, No. 4, 2001
RODINIA, GONDWANA AND ASIA
Major Precambrian Events in Northwestern China Songnian Lu Tianjin Institute of Geology and Mineral Resources, CGS, China Precambrian rocks in China are mainly exposed in the North China Platform. They also occur at the margins of the Tarim Platform, the Qaidam Block and the South China Platform. Although several studies have been carried out on Precambrian geology of North China Platform, data on Northwestern China are sparse. Based on our recent investigations and new isotopic ages, the oldest granitic gneiss is identified at the Altyn tagh fault belt, and the Neoproterozoic thermo-tectonic events are very significant. The area preserves records for terminal stages of both Archean and Paleoproterozoic in south and northwest China, and marks an important link in the evolution from Rodinia to Gondwanaland supercontinents. This work introduces the major Precambrian events in Northwestern China. The first and foremost is the identification of oldest rocks from the Altyn tagh, which is located in the eastern margin of Tarim Platform. According to previous data, the Milan Group is composed of quartz-feldspar gneisses and belongs to the Neoarchean, with ca. 2500 Ma U-Pb isotopic age. Our recent study shows that the Milan Groupconsists mainly of TTG and granitic gneisses, with only minor supracrustal components. Single zircon U-Pb age of granitic gniess yields 3605 k 43 Ma. Ages of 2500 Ma, 1900 Ma and 1100 Ma were also measured from the same sample, although 3600 Ma age preserved by zircons is significant. The Archean rocks of Kuruktag and Dunhuang regions in the Tarim Platform comprise tonalite, trondhjemite and granodiorite gneisses. Amphibolites occur as enclaves within the gneisses. A U-Pb age of 2582 11Ma was reported from the granitic gneiss at Kuruktag. The tonalitic and trondhjemitic gneisses yield upper intercepts U-Pb age of 2670 12 Ma, and lower intercept age of 1017 k 3 Ma. The former should represent the crystallization age and the latter might reflect the age of Neoproterozoic event. 2.68 Ga Nd model age from the same sample, which is similar to the above U-Pb age, reflects an important crust-forming event in this area. Paleoproterozoic and Mesoproterozoic rocks are largely preserved in the northern margin of Tarim, southwestern segment of North China Platform, Central Qilian and North Kulun Mountain. U-Pb isotopic age from the mylonitised granites here indicates 1499 21 Ma. Another sample of granitic gneiss from the northern part of East Kulun Mountain yields similar age of 1624 & 22 Ma. The granitic gneisses intruded into the Paleoproterozoic Jinshikou Group, which reflect an important magmatic activity at the end of Paleoproterozoic. Neoproterozoic rocks are widespread in the study area represented mainly by vast Neoproterozoic granitic gneiss, with supracrustal rocks preserved as enclaves or relicts. The protolith of these gneisses include a variety of granites. Ages of 1020 +. 41 Ma and 803 7 Ma have been obtained from trondhjemite
+-
and granodiorite gneisses, respectively. An age of 744 & 28 Ma is also measured from a K-rich granitic gneiss sample, which may reflect the time of a late or post orogenic event. The Quanji Group as the first sedimentary cover rests unconformably upon the metamorphic Dakendaban Complex. 738 +. 28 Ma from single zircon U-Pb dating is obtained from volcanic rocks of this group. Based on a series of geochronological data, the unconformity between the Quanji Group and Dakendaban Complex is placed at ca. 750-800 Ma ago. Eclogites within the Neoproterozoic granitic gneiss belt yield Sm-Nd mineral and whole rock isochron age of 545 5 8 Ma. U-Pb dating of eclogites in Yuqiahe and Shaliuhe shows 499.6 -t 6.5 Ma. On basis of the above ages from different methods, a significant Pan-African tectonothermal event between 550 Ma-500 Ma can be identified in this terrain, linking with similar events in Gondwanaland. Eight zircon grains from the host gneisses of the Liuyuan eclogites define a discordia line with an upper intercept age of 880 F 31 Ma. T,, age is about 1500 Ma, which is much older than 880Ma of U-Pb age of the gneiss. An eclogite sample from Gupuqian area yields upper intercept age of 861 k 71 Ma and lower intercept age of 440 k 29 Ma, suggesting two important events. Neoproterozoic pegmatite swarms are also preserved in the Altyn tagh fault Belt. Zircons from the pegmatite yield a U-Pb age of 913 +. 26 Ma, which illustrates that the pegmatite swarms were emplaced during the Neoproterozoic magmatic activity. Recently discovered eclogites in the southwestern segment of Altyn tagh gave a U-Pb zircon age of 504 & 5 Ma. High-pressure metamorphosed pelitic rocks in the central part of the fault belt show 575 k 3 Ma from Ar-Ar dating of phengite. We identify some similarity between northern a n d Northwestern China in the 3600 Ma magmatism and thermotectonic events a t terminal stages of Archean a n d Paleoproterozoic. However, the extensive development of Neoproterozoic events is largely confined to Northwestern China. The salient findings from our study are summarised as follows. (1)3600 Ma zircons were identified in the Altyn tagh fault belt, which represents the third oldest isotopic age record from China. (2) TTG and K-rich granitic gneisses developed in the terminal stages of Archean and Paleoproterozoicrespectively in both north and northwest China. (3) The granite emplacement is concentrated at the early Neoproterozoic period in Northwestern China. These are developed in the southern part of the north margin of the Qaidam basin, which may represent an important tectonic boundary. (4) The isotopic ages of ca. 550-500 Ma recorded from eclogites and other high-pressure metamorphic rocks in the study area may correspond to the imprints of PanAfrican orogeny in this area, which needs detailed evaluation in future studies.
Gondwnnn Rescorch, V. 4, No. 4, 2001