Middle Devonian Picrites of the Southern Margin of Altay Orogenic Belt and Implications for the Tectonic Setting and Petrogenesis

The Altay orogenic belt of Xinjiang in NW China represents one of the important sites of juvenile crustal growth during the Phanerozoic. However, some important issues, e.g., tectonic evolution and petrogenesis, still remain controversial. The picrites in the south margin of the Altay orogenic belt were discovered in the lower part of marine volcanic-sedimentary sequences of the northwest-striking Middle Devonian Beitashan Formation (Fm.), which consists chiefly of intermediate-basic volcanic rocks intercalated minor carbonate, siltstone and siliceous rocks. The picrites are usually highly porphyritic, and contain abundant forsteritic olivine phenocrysts with minor clinopyroxene distributed in the groundmass, which consist of olivine, clinopyroxene and plagioclase with minor Fe-Ti oxides. The MgO contents of the picrites range from 14 wt% to 22 wt% with Mg# (atomic Mg/(Mg+Fe) ratio) of 0.75-0.80. They are characterized by slightly negative Ti anomalies, remarkably negative Nb and Ta anomalies and slightly positive P and Sm anomalies with the similar abundances of HFSE as MORB on the MORB-normalized trace element patterns, all of which characterize typical island arc magmas. In combing with the southwestward migration of the magmas of the Beitashan Fm., we propose that the magmas may result from the southwestward subduction of Junggar ocean plate. The Zr/Nb ratios (23-66) of both picrites and basalts resemble the MORB (10-66), suggesting that they were derived from the MORB-like sources. However, the basalts and picrites display some distinguishable element ratios and REE patterns, e.g., Ti/V (23-35) and Zr/Sm (18-23) ratios of basalts are higher than those of picrites (14-17 and 14-15 respectively), and the basalts display flat-type REE-chondrite patterns whereas the picrites are characterized by lower total REE concentrations ((26-34)×10^-6) and slight enrichment of light REE. These distinguished geochemical characteristics could be interpreted by different partial melting degrees and mantle sources, i.e., the basalts were generated by lower partial melting of amphibole-bearing spinel peridotite which was metasomatized by fluids released from subducted oceanic crust, and the picrites were resulted from the higher degree of partial melting of metasomatized garnet peridotite under high temperature. In contrast, the andesite with significant LREE and LILE enrichment may be resulted from the partial melting of eclogites.

Growth Dynamics of the Middle Devonian Jukoupu Bioherm,Xinshao, Hunan

The growth process, palaeoecological features and the function of organisms in reef-building of the Jukoupu bioherm in Hunan are discussed in detail. The bioherm is divided into 10 microfacies. Organisms are very abundant in the reef. Thirty-two genera Belonging to 7 phyla have been recognized. They make up 5 communities composed mainly of stromatopoids, Corals and algae, each of which has its own salient ecological features. An analysis of all communities shows that the organic functions are very complex. The same organisms may possess diverse functions simultaneously as the reef growth enters the late stage. The growth process of the reef may involve five stages' (l)biostrome stage, (2) stabilization stage, (3) frame-type reef-building stage, (4) binding and covering-type reef-building stage, and (5) baffle- type reef -building stage.

The Middle Devonian Bimodal Association of Volcanic Rocks in the Northern Area of East Junggar, Xinjiang

The Middle Devonian volcanic rocks in the northern area of East Junggar, located between the Ertix andUlungur rivers of northern Xinjiang, may be divided into basic and acid ones. It is evident that a compositionalgap exists between the two groups so that the volcanic rocks are not in line with a calc-alkaline series becausethe intermediate rocks are absent in the area. The fact shows that the volcanic rocks are a typical bimodal asso-ciation. The formation of the bimodal association of volcanic rocks in the area was closely related to continen-tal rifting or continental extension in the Middle Devonian. In such a tectonic setting, magmas were first pro-duced by partial melting of the mantle. Where crustal thinning was greater, the magmas ascended and eruptedon the surface directly so that the basic volcanic rocks formed, but olivine and/or partial pyroxenefractionation occurred in the magmas during their ascent through the thinning crust. On the other hand, wherecrustal thinning was less, ascending mantle-derived magmas reached the lower crust and accumulated there, re-sulting in partial melting of the lower crust and thus giving rise to the contaminated magma which was consoli-dated as acid volcanic rocks on the surface.

A New Bothriolepid Antiarch from the Middle Devonian in Luquan,Yunnan,South China and Its Stratigraphic and Biogeographic Significances

Chahuaqingolepis magniporus gen.et sp.nov.,a new taxon of Bothriolepididae(Placodermi:Antiarcha),is described from the Middle Devonian in Luquan County,Yunnan Province,South China.The phylogenetic analysis shows that Chahuaqingolepis is the sister group of Wufengshania with the synapomorphy of the large obtected nuchal area.Together with the findings of other associated antiarchs including Dianolepis,Bothriolepis,and Hunanolepis,the new bothriolepid antiarch is assigned to the Bothriolepis sinensis-Hunanolepis fish assemblage,which characterizes the Eifelian of the Middle Devonian in South China.The fish-bearing Tanglishu Formation can be well correlated with the Tiaomajian Formation in Changsha,Shangshuanghe Formation and lower part of the Haikou Formation in Qujing,and Laohutou Formation in Shaoguan and Lechang areas.This discovery not only enriches the Middle Devonian diversity of bothriolepids,but also provides reliable paleoichthyological evidence on the division and correlation of the fish-bearing Devonian strata in South China and deepens the understanding of the biogeography of the Devonian bothriolepids.

Paleocommunity Replacements of Benthic Brachiopod in the Middle-Upper Devonian in the Longmenshan Area,Southwestern China:Responses to Sea Level Fluctuations

Twelve paleocommunities dominated by benthic brachiopod are recognized in the Givetian-Frasnian stages of the Devonian in the Longmenshan area, southwestern China, in which two kinds of brachiopod community replacement are classified. One is the abrupt replacement, represented by abrupt alternation between the Leiorhynchus community and Zhonghuacoelia-Striatopugnax community in the Frasnian Tuqiaozi Formation. The other is the gradual one, developed in the Givetian Guanwushan Formation, which had been completed by the shift of the Independatrypa lemma-Uncinulus heterocostellis-Emanuella takwanensis community via the Sinospongophyllum irregulare-Pseudomicroplasma fongi community to the Clathrocoilona spissa-Hexagonaria composite reef community. According to analyses of the paleocommunities, either the abrupt or gradual paleocommunity replacement of the Middle-Upper Devonian in the Longmenshan area is suggested as a response to the 5th-order sea level fluctuation due to the replacements of the paleocommunities in a tracts-system of depositional sequence. It is supposed that changes of paleocommunity diversity, one of the results of paleocommunity replacement, are depended on the range and magnitude of sea level fluctuation, but there is not a linear relationship between them. Furthermore, a suggestion is proposed that the concept of paleocommunity succession seem to be abandoned in the paleocommunity analysis because it almost never be practiced to recover the information of community succession in the geological record at present.

Genesis and characteristics of the dolomite reservoirs in Middle Devonian Guanwushan Formation, Northwest Sichuan Basin, SW China

A great breakthrough has been made on oil-gas exploration of the Middle Devonian Guanwushan Formation in Northwest Sichuan Basin.The further exploration shows that the dolomite reservoir is complicated with insufficient investigation.Taking the Middle Devonian Guanwushan Formation as an example,systematic study has been carried out on its reservoir character,mechanism and genesis of dolostone.The field and core data suggest that reservoir types of Guanwushan Formation consist of crystalline dolomite,breccia dolomite and reef dolomite,and reservoir space is characterized by intercrystal(dissolution)pores,residual pores among breccias,intragranular dissolution pores,pockets as well as fractures.Besides,pore structure is of good configuration,showing medium-low porosity and moderate permeability,with average porosity and permeability of 2.23%and 0.44mD respectively,which suggesting fracture-pore and fracture-cavity reservoir.Combined with the previous research,it is suggested that the reservoir of Guanwushan Formation was controlled by facies,dolomitization and two-step karstification,with reef-shoal facies being material base for reservoir development,early dolomitization being key factor for reservoir space preservation,polyphase karstification being foundation for reservoir development and tectogenesis finally finalizing the reservoir.

SHRIMP U-Pb Zircon Dating of the Tula Granite Pluton on the South Side of the Altun Fault and Its Geological Implications

The Tula A2-subtype granite pluton is located between the Altun fault and its branching fault. According to the geological, geochemical, REE and trace elements characteristics, it belongs to the A2 （PA） subtype granite. The SHRIMP U-Pb zircon dating gives a result of 385.2±8.1 Ma, which is located between the Middle and Late Devonian in the international stratigraphic chart, and can be regarded as the crystallization age of the Tula granite. The study indicates that the Tula area was in a local extensional environment in the end of the Middle Devonian, and that environment was probably related to the synchronized strike-slip activity of the Altun fault.

Age of Berh Hairhan Formation of Mongolia dated by conodonts

Conodont Caudicriodus angustus angustus （Steward et al. , 1956 ） and Catuticriodus angustus cauda （Wang et al. , 2005） were found from a big limestone lens of the Berh Hairhan Formation in the place of W 150 km to Ulaanbaatar City. The discovery indicates that the Beth Haihan Formation should be assigned to Middle Devonian Eifelian Stage. This conodont fauna was from a shallow cooler water facies in the relatively high latitude.

The reservoir characteristics research of Devonian Jinbaoshi Formation,Northwest Sichuan Basin,SW China

A great breakthrough was achieved on the Middle Devonian's oil-gas exploration in the Longmenshan area in northwestern Sichuan.However,only the Guanwushan Formation was researched in detail.The underlying formation-Jinbaoshi Formation as a set of the potential well reservoir and its reservoir characteristics have not been discussed.Based on the section's fine description,thin section analysis,and core borehole logging,combined with petrophysical analysis,the reservoir characteristics,controlling factors,and evolution process of the Jinbaoshi Formation reservoir in the Longmenshan area are deeply studied in this paper.The results showed that the Jinbaoshi Formation has developed a set of sand-shale interbedding deposit of terrigenous clastic and carbonate rocks.The reservoir lithology is mainly quartz sandstone and calcareous quartz sandstone.The quartz sandstone reservoir has good physical properties,but the calcareous quartz sandstone has the characteristics of strong heterogeneity,low porosity,and permeability.Residual intergranular pores are mainly developed in the reservoir space.And the development of intercrystalline pores and intergranular dissolved pores can also be seen,but the development frequency is low,and the scale is small.Among them,the Devonian Jinbaoshi Formation quartz sandstone's average porosity is about 8%,and the highest is 18.43%.The main diagenesis types of the Jinbaoshi Formation are compaction,recrystalization,and dissolution.Compaction is the main reason for the destruction of original pores,and dissolution is the main driving force for later porosity and permeability improvement.Rock fragments,calcareous silty clasts,and bioclastic grains are the main dissolution objects.The formation of intergranular dissolved pores significantly improves the physical properties of the reservoir.Therefore,the reservoir development of the Jinbaoshi Formation is mainly controlled both by favourable sedimentary facies and diagenesis.Quartz sandstone and calcareous sandstone of the Jinbaoshi Formation were controlled by early sedimentary facies in the syngenetic rock stage and developed more early intergranular pores;in the early diagenesis stage,compaction-pressure solution and cementation resulted in residual intergranular pores.In the middle-late diagenesis stage,the calcareous cement is dissolved,and more pores were formed.

Discovery of late Paleozoic fossils in tectonic blocks of the Dabao Formation in the South Qinling Orogenic Belt and their significance

The Dabao Formation in the South Qinling Orogenic Belt was previously regarded as Ordovician in age and consists of clastic matrix and blocks of siltstone,limestone,chert,and volcanic rocks.However,some Middle Devonian corals,conodonts,and other fossil fragments within the limestone blocks were discovered in recent field investigations,indicating that the Dabao Formation was formed during late Paleozoic.Combined with other regional geological data,the Dabao Formation in the Southern Qinling Orogenic Belt is considered to be a late Paleozoic or early Mesozoic accretionary complex.