GR

Neoproterozoic Orogeny in Northwestern China

Lu Songnian, Li Huaikun, Yu Haifeng, Zhao Fengqing and Yang Chunliang

Tinnjin lnstitzite of Geology and Mineral Resources, CAGS, No.4, 8th Xond, Dnzlzigu Tianjirz 3001 70, China

The Qilianshan Mountain in Northwestern China is commonly regarded as the Caledonian orogenic be1 t. A very complex evolution of this belt has been described in many Chinese literature (Chen et al., 1995; Xia et al., 1996). The ophiolites of Cambrian-Ordovician ages and pre-Late Devonian unconformity were discovered. Recently an eclogi te-granitoid gneiss belt, which is situated along the northern margin of the Qaidam Basin, to the south of the Qilianshan belt, was also reported (Yang et al., 1998; LU et al., 1999).

Earlier investigations confirm that a Paleoproterozoic metamorphic unit, Mesoproterozoic sedimentary rocks and Paleozoic volcano-sedimentary deposi ts have developed in this belt (Qinghai Bureau, 1991). However, a new stratigraphic and tectonic framework is now proposed on the basis of recent investigations. In the study area occupied by the so-called "Paleoproterozoic Dakendaban Group", the major rock types found are granitoid gneisses. In fact, these granitoid gneisses are predominant within the "group" and only sinall outcrops of suyracrustal rocks are preserved in the gneisses as enclaves or relicts. So we prefer to call them as "Complex", not "Group". The area occupied by the Dakendaban Complex is scattered over a length of 800 km from southeast to northwest with a width of up to 50 km. The granitoid gneisses of Dakendaban Complex include mainly three rock types: tonalite, monzonite-granite and K- rich granite. A typical coesite-eclogite outcrop has been discovered recently on the Yuqiahe River of this region (Li et al., 1999). The eclogite occurs lenticularly in the granitoid gneiss of the Dakendaban Complex. The eclogite consists predominantly of garnet and omphacite with minor quartz/ coesite, muscovite/phengite, hornblende and rutile. The garnet is composed of 51-59% almandine, 26-31% pyrope and 13-19'/0 grossular. The omphacite contains 45-48% jadeite. A little amount of phengite, with abnormal high B, value of 9.075A, occurs in the eclogite. The hornblende is the ultra-high pressure type, i.e. barroisite. The most important aspect is the finding of coesite in the eclogite.

The eclogitc retains its original characteristics well, and has only suffered slight retrograde metamorphism. There are three generations of minerals in the eclogite: (1) the peak metamorphic mineral assemblage (the earliest generation): garnet + omphacite + muscovite/phengite + quartz/coesite + rutile; (2) the retrograde metamorphic mineral assemblage: barroisite + worm-like quartz; and (3) the post- metamorphic stage represented by muscovite. The dominant mineral assemblage in the rock is that which represents the metamorphic stage. The eclogite is inferred to be of ultra-high pressure (>2.SGPa) to high pressure (1.7- 0.73 GPa). It might be concluded that the northern margin of the Qadam Basin was a major convergent zone of ancient continental plates.

A series of geochronological data for the granitoid rocks and eclogites have been obtained. According to these new isotopic data, there is no evidence to support that the dominant rocks of the Dakendaban Complex belongs to the Paleoproterozoic time. The new data indicate that the emplacement of the granitoid rocks took place from the late Mesoyroterozoic (1020k41 Ma) to early Neoproterozoic (803+7 Ma). An age of 763f10 Ma has been measured from an eclogite sample. It is reasonable to suppose that there is a Neoproterozoic magmatic suture between the Qaidam and Qilianshan terrains. Meanwhile, a U-Pb age of 680f26Ma from volcanic rocks has just been obtained. The volcanic rocks are preserved in the lower part of the subsequent rifting sequence. Thus, both collision and rifting events of Neoproterozoic age have been revealed from Northwestern China.

Neoproterozoic thermotectonic events have been reported from many places of the belt. Among these events, the most important are the major magmatic activity and the occurrence of eclogites as enclaves in granitoid gneisses of Neoproterozoic age. The d t rahgh pressure metamorphic record of the Neoproterozoic is recognized within the Caledonian Orogen, which shows that a Neoproterozoic orogeny was developed. After the collision at the early

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Neoproterozoic time, subsequent rift-related deposits were formed. The mobile belts from the Sinian to the early Palaeozoic developed.

Synthesizing the above results, we come to the conclusion that the following points are to be recognized for future work:

1. The extensive magmatic zones of Neoproterozoic are preserved in the young Orogen. Plutonic rocks of different tectonic types including subduction, collision and post- orogenic granite types are developed in the zones. Geochronology, geochemistry and tectonic origin of plutonic rocks need to be studied in detail.

2. As enclaves in granitoid gneisses, eclogites have been discovered in a few places of the orogen. Some of the eclogites are products of the Neoproterozoic collision between terrains, based on present isotopic data. The eclogite-bearing granitoid gneisses show evidence of collision belts or plate margins of the Neoproterozoic age.

3. Besides the collision event, subsequent rift basins have developed during the Neoproterozoic time in the orogen. A preferred age of the transitional time between collision and rifting is concentrated at about 700-800 Ma.

References

Chen Bingwei, Wang Yanbin and Zuo Guochao (1995) Terrain subdivision of the northern Qinghai-Xinjiang (Tibet) Plateau and its tectonic evolution, Acta Geophys. Sinica, v. 38, Supplement 11, pp.98-113.

Li Huaikun, Lu Songnian, Zhao Fengqing , Yu Haifeng and Zheng Jiankang (1999) The determination and significance of the coesite eclogite on the Yuqia River on the North margin of the Qadam Basin. Geosci. J., Graduate School, China Univ. of Geosci., v. 13, pp. 43-50.

Lu Songnian, Zhao Fengqing, Mei Hualin, Yu Haifeng, Li Huaikun and Zheng Jiankang (1999) Discovery and significance of eclogite-granitoid belts in Northwest China, Gondwana Res., v. 2, pp.137-138.

Qinghai Bureau of Geology and Mineral Resources, Regional Geology of Qinghai Province (1991) Beijing: Geol. Publ. House, pp.1-662.

Xia Lingqi, Xia Zuchun and Xu Xueyi (1996) The origin of marine volcanic rocks in the North Qilianshan Mountain, Beijing: Geol. Publ. House, pp.1-153.

Yang Jingsui, Xu Zhiqin, Li Haibing, WuCailai, Cui Junwen, Zhang Jianxin and Chen Wun (1998) Discovery of eclogite in Chaibeiyuan Area, Western China. Chinese Sci. Bull., V. 43, pp.1544-1549.

Gondzuana Research, V. 2, No. 4, p p . 611-613. 0 1999 International Association for Gondzuann Research, Japnn. ISSN: 1342-937X GR

Goizdwni~n Resenrch

On the Boundary Between Gondwanaland and Eurasian Continent Seen in China

Wan Tianfeng

China University of Geosciences, Beijing 200083, China

Introduction

The problem of boundary between the Gondwanaland and Eurasian continent was often discussed in geodynamics. Usually, the boundary between Gondwanaland (i.e. Indian- Australian plate) and Eurasian continent in Neogene was recognized at the Main Boundary Thrust of Himalayan and Sunda Trench (q, Fig. 1, Huang and Liu, 1991; Chen and Xie, 1994; Metcalf, 1995). The boundary between Gondwanaland and Eurasian continent in Late Cretaceous and Early Eogene was determined at the Bangong- Nujiang suture zone (r, Fig. 1, Huang and Liu, 1991; Chen and Xie, 1994; Metcalf, 1995). The boundary between Gondwanaland

and Eurasian continent in Triassic (i.e. Indosinian event) was discussed in the IGCP Project 321 symposia many times (Chen and Xie, 1994; Liu et al., 1991; Metcalf, 1995; Wan, 1997), and is observed to be situated at the Kunlun - Hoh Xi1 shan - Changning-Menglian zone, near the Lancangjiang - Middle Thailand - Middle Malaysia Peninsula (t, Fig. 1).

The boundary between Gondwanaland and Eurasian continent in the Paleozoic is enigma tic due to the similar fossil record, identical paleoecological environment and contradictory paleomagnetic data. The characteristics of Sinian system (800 -560 Ma) is however useful to determine the boundary between Gondwanaland and Eurasian continent, which will be discussed in this paper.