Liu Y.Q.,Ghinese Academy of Geological science |
Kuang H.W.,Ghinese Academy of Geological science |
Kuang H.W.,Yangtze University |
Peng N.,China University of Geosciences |
And 3 more authors.
Acta Petrologica Sinica | Year: 2010
Molar-tooth structure; carbonates were widely distributed in the Proterozoic cratonic carbonates worldwide and well preserved within the Mesoproterozoic and Neproterozoic stratigraphy in the vast areas of China as well. Microspar structure, no matter temporal and spatial distribution, displays a consistency of sedimentology and petrology. Molar tooth structure (MTS) , represented by complex ptygmatical shapes, filled by fine (5 - I5n,m) , equant spam' calcite ( or dolomite). The host rocks is mainly fine-grained carbonates with a few terrigenious elastics. Images of cathode luminescence (CL) and backscattering (BS) of microspar structure show that a rhombus crystal nucleus exit in the interior of the individual microspar calcite and the stacked equant calcite spam' surrounding the nucleus, which suggests that microspar spray calcite were constructed by an aggravated accretion of calcite microspar surrounding an initial nucleus of calcite. Elements content of microspar and host rocks by electron probe analysis indicates that Ca content be up to 38% -40% , the values of Mg, Al, Si, P, K and Fe lower than 0. 2% , and clay mineral content and Si, Al is a little higher. The host rocks of microspar is mainly composed of fine-grained carbonates ( calcarenite, calcisiltite, calciliitite) and argillic and iron caicilutite. Vertical succession of microspar structure carbonate shows a deposition stacked by a shallowing-upward subtidal and peritidal cycles. An individual cyclic sequence is characterized by thinning and shallowing-upward. Numerous erosional surfaces and truncated and fills or guttered bases, hardground and stylolite structures, a variety of horizon, oblique, wave, graded beddings and deformed or subaerial exposed structures are common within deposition, which all suggest that a indicative of shallow subtidal and supratidal environments and a sedimentary settings constraint of Molar-tooth structure. In fact, for microspar structure carbonates, it is not only a sedimentary indicator of peritidal products, but also an important and particular carbonates in which Proterozoic global paleogeography and paleogeochemistry were recorded. Microspar structure carbonates is also especially benefit a global change and the reconstruction of the Proterozoic paleogeography and tectonic.
Zhang J.X.,Ghinese Academy of Geological science |
Meng F.C.,Ghinese Academy of Geological science |
Yu S.Y.,Ghinese Academy of Geological science
Acta Petrologica Sinica | Year: 2010
North Qilian (NQL) -North Altiin (NAT) HP/LT metamorphic and North Qaidam (NQD) -South Altiin (SAT) UHP metamorphic belt arc located in the northern and southern sides of Qilian-Altun orogen, respectively. NQL-NAT HP/LT metamorphic belt mainly consists of hlueschist, low-temperature eclogite and high pressure metasedimentary rocks. Eclogites were formed under the metamorphic conditions of 420∼570°C and 2. 1∼2. 6GPa, and at 510-440Ma. The lawsonite-hearing eclogites and carpholite-hearing high pressure metasdimentary rocks indicate the existence of cold suhduction zone gradient and cold suhduction brought a large amount of H20 to the deep mantle in the Early Palaeozoic times. HP/LT metamorphic belt is associated with ophiolite, subduction-accretion complex, island arc and back-arc basin, suggesting that the NQL is a typical Early Paleozoic acccretion orogenic belt. In contrast, the NQD-SAT UHP metamorphic belt is composed of eclogite, garnet peridotite, high pressure granulite and continental orogneiss and paragneiss. These rocks were subjected to UHP metamorphism at T > 700T! and P > 2. 7GPa. Geochronology suggests that the UHP metamorphism occurred at 500 to 420Ma, and the protolith age of eclogite at 850-750Ma, related to a Neoproterozic continental rift origin. Field relationship, petrology and geochronology suggest that the NQD-SAT UHP metamorphic belt was resulted from the deep subduction of continental crust rocks. In the NQD-SAT UHP metamorphic belt, UHP eclogite and HP granulite produced in different thermal environments at same time, suggesting a "paired metamorphic belt" related to continental subduction and collision, and also implying that the NQD-SAT is a typical Early Paleozoic collision orogen. These data indicate accretion orogeny and collision orogeny developed synchronously in two side of Qilian-Altun orogenic belt, and formed a composite orogen as a result of different type of orogeny. HP/LT metamorphic belt and HP/UHP metamorphic belt control the orogenic type and mechanism of Qilian-Altun orogen in Early Paleozoic time.