Hyperpycnal-fed turbidite lobe architecture and recent sedimentary processes: A case study from the Al Batha turbidite system, Oman margin

The main sediment depocenter along the Oman margin is the Al Batha turbidite system that develops in the Gulf of Oman basin. It is directly connected to the wadi Al Batha, and forms a typical sand and mud rich point source system that acts as regional sediment conduit and feeds a ~ 1000 km² sandy lobe.The Al Batha lobe depositional architecture has been investigated in detail using very high-resolution seismic, multibeam echosounder data and sediment cores. Several scales of depositional architecture can be observed. The Al Batha lobe is composed of several depositional units, made of stacked elementary sediment bodies (thinner than 5 m) that are each related to a single flow event. The lobe is connected to the feeder system through a channel-lobe transition zone (CLTZ) that extends on more than 25 km. The lobe can be divided into proximal, middle and distal lobe areas. The proximal lobe is an area of erosion and by-pass with small axial feeder channels that rapidly splay into several small distributaries. They disappear in the mid-lobe area where deposits consist of vertically stacked tabular to lens-shaped sediment bodies, with a lateral continuity that can exceed 10 km. The distal lobe fringe shows a classical facies transition towards thin-bedded basin plain deposits.Sub-surface deposits consist of sandy turbidites and hyperpycnites, interbedded with fine-grained deposits (thin turbidites, hyperpycnites, or hemipelagites). Although these distal deposits are mainly related to flow transformations and concentration evolution, they highlight the importance of flooding of the wadi Al Batha on the sediment transfer to the deep basin. The thick sandy hyperpycnites recovered in such a distal area are also possibly related to the initial properties of gravity flows, in relation to the flooding characteristics of mountainous desert streams.Finally, the Al Batha lobe depositional architecture is typical of sand-rich lobes found within “small”, sand and mud rich turbidite systems fed by mountainous “dirty” rivers. Turbidite sedimentation in the Al Batha system appears to be primarily controlled by the strong climatic and geomorphic forcing parameters (i.e. semi-arid environment with ephemeral, mountainous rivers subjected to flash-flooding).     

Middle Permian cephalopods from central Peninsular Malaysia: implications for faunal migration through the southern Tethys

A Wordian (Middle Permian) cephalopod fauna consisting of four ammonoid species, Tauroceras aff. scrobiculatum (Gemmellaro), Agathiceras sp., Bamyaniceras orientale n. sp. and Pronoritidae gen. and sp. indet., and two nautiloid species, Tainoceras sp. and Orthocerida fam. indet., were recovered from the Bera South area, southern Pahang, Peninsular Malaysia. The outcrop represents the southernmost extent of the Bera Formation. Bamyaniceras and Tainoceras are recorded in Malaysia for the first time. The presence of T. aff. scrobiculatum suggests a regional correlation with an ammonoid-bearing bed of Sungai Cheroh, western Pahang, and permits global correlations with strata of northeastern Iraq, northern Oman and Sicily (Italy). A possible eastwards faunal migration through the southern Tethys is suggested.     

新疆塔里木盆地哈拉奇地区晚石炭世一早二叠世哈拉奇组的建立

哈拉奇组为新疆阿合奇县开展1：5万区域地质矿产调查工作时建立的岩石地层单位,分布在新疆阿合奇县哈拉奇地区北部,主要为一套深水碳酸盐沉积,与该区南部属碳酸盐台地沉积的康克林组在岩性、岩石组合及古生物组合上形成明显的差别。通过对该组重力流成因的生物屑砾屑灰岩中古生物化石的研究,确定其筵带为Triticites带和Pseudoschwagerina带,进而确定该组地质时代为晚石炭世-早二叠世,与南区康克林组为同时异相沉积。哈拉奇组的建立完善了该地区上石炭统一下二叠统的地层系统,确定了该地区晚石炭世-早二叠世台地边缘斜坡相的存在,对塔西北地区晚石炭世-早二叠世岩相古地理研究具有重要意义。     

Geobiological interpretation of the oxygen-deficient deposits of the Middle Permian marine source rocks in South China:A working hypothesis

To decipher the origin of oxygen-deficient shelfal deposits is significant for tracing the distribution of marine source rocks and interpreting the evolution of depositional environment. The origin of the Middle Permian Chihsia Formation in South China remains a puzzle for long with its evident oxygen-deficient features but diverse benthos. This paper shows a typical Chihsian depositional rhythm composed of the massive and the laminated limestones with ecological and geochemical features. Massive bioclastic limestone from the rhythm was aerobic in paleoxygenation condition indicated by both the ecological and geochemical features. However, a contradictory oxygenation was inferred for the “laminated” counterpart from the rhythm, with the ecological signal being aerobic and the geochemical one being anoxic. The difference in ecological and geochemical indications was interpreted as the instability of paleoxygenation condition in shelf environments, caused by an enhanced paleoproductivity. Rhythmic occurrence of the oxygen-deficient condition might have been stemmed from paleo-Tethyan paleocurrents flowing across South China. __________ Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6): 789–796 [译自: 地球科学—中国地质大学学报]     

The geochemistry of mantle chromitites from the northern part of the Oman ophiolite: inferred parental melt compositions

Chromitites from a single section through the mantle in the Oman ophiolite are of two different types. Low-cr# chromitites, of MORB affinity are found in the upper part of the section, close to the Moho. High-cr# chromitites, with arc affinities are found deeper in the mantle. Experimental data are used to recover the compositions of the melts parental to the chromitites and show that the low-cr# chromitites were derived from melts with 14.5–15.4 wt% Al₂O₃, with 0.4 to 0.9 wt% TiO₂ and with a maximum possible mg# of 0.76. In contrast the high-cr# chromitites were derived from melts with 11.8–12.9 wt% Al₂O₃, 0.2–0.35 wt% TiO₂ and a maximum melt mg# of 0.785. Comparison with the published compositions of lavas from the Oman ophiolite shows that the low-cr# chromitites may be genetically related to the upper (Lasail, and Alley) pillow lava units and the high-cr# chromitites the boninites of the upper pillow lava Alley Unit. The calculated TiO₂–Al₂O₃ compositions of the parental chromitite magmas indicate that the high-cr# chromitites were derived from high-Ca boninitic melts, produced by melting of depleted mantle peridotite. The low-cr# chromitites were derived from melts which were a mixture of two end-members—one represented by a depleted mantle melt and the other represented by MORB. This mixing probably took place as a result of melt–rock reaction. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Early and Middle Permian paleoclimates of the Baoshan Block, western Yunnan, China: insight from carbonates

In Permian times the Baoshan Block of western Yunnan, southwest China formed the eastern part of the Cimmerian Continent. Most biogeographical and sedimentological data indicate that the Early Permian Dingjiazhai Formation formed on the block under conditions strongly influenced by the Permo-Carboniferous glaciation. After Early Permian rifting, with post-glaciation climatic amelioration, and as the Baoshan Block drifted northwards to approach South China and Indochina, faunal elements characteristic of Gondwana affinity decreased, while those of Cathaysian affinity increased. Finally, Late Permian faunas are characterized by exclusively Cathaysian elements. This shift of marine provinciality becomes an important indicator in understanding the Permian paleoclimatic evolution of the region. This research investigated the composition of carbonate grain associations and the early diagenetic features of limestones from the upper part of the Dingjiazhai Formation, and from the overlying Yongde and Shazipo formations. A sharp distinction in petrological and diagenetic features is recognized between the Dingjiazhai Formation and the two overlying formations. The Dingjiazhai carbonates are characterized by the bryonoderm (bryozoan-echinoderm)-extended facies of the heterozoan association, with no non-skeletal grains. Because early diagenetic cement was rarely formed, the Dingjiazhai carbonates experienced strong diagenetic compaction. In contrast, the Yongde and Shazipo carbonates show a chloroforam facies of photozoan association, with the common occurrence of non-skeletal grains. These carbonates were well cemented during early diagenetic processes. From comparison with Permian cool-water carbonates from northern Pangea and Tasmania, Australia, the Dingjiazhai carbonates are interpreted as deposits of warm-temperate conditions, while the overlying carbonates are considered to be deposits of subtropical or tropical conditions. This climatic interpretation, based on the petrographic features of the Permian carbonates, agrees well with existing biogeographical data from the region.     

SHRIMP U-Pb zircon geochronology and Hf isotope analyses of Middle Permian–early triassic intrusions in southern Manzhouli area,Northeast China: implications for the subduction of Mongol-Okhotsk plate beneath the Erguna massif

ABSTRACT

The results of SHRIMP U-Pb ages and in situ Hf isotope of zircons from three granites in the southern Manzhouli region of northeast China, provide new data to understand the subduction process of Mongol-Okhotsk Plate beneath the Erguna massif. SHRIMP U-Pb zircon geochronology results yield an age of 265.5 Ma (middle Permian) for fine-grained monzogranite. Rocks from the Early–Middle Triassic are mainly granodiorite (247.4 ± 4.6 and 249.3 ± 4.9 Ma), the granites are with SiO₂ = 60.0–77.4 wt.%, Al₂O₃ = 12.3–16.8wt.% and Na₂O/K₂O = 0.7–1.9. Chemically, they are metaluminous to peraluminous and belong to the high-K calc-alkaline series. Enrichments in the large ion lithophile elements (e.g., Rb, Ba, and K) and depletions in the high field strength elements (e.g., Nb, Ta, and Ti) are typical for these rock types. The monzogranite (~265 Ma) and granodiorite (~247 Ma) contain zircons with εHf(t) values of 6.3–8.5 and 5.1–7.9, yielding T_DM2 model ages of 888–752 and 958–774 Ma, respectively. These geochemical and zircon Hf isotopic data indicate that primary magmas for Middle Permian–Early Triassic granites crystallized from primary magmas generated by Neoproterozoic crustal materials, formed in an active continental margin setting. The andesite of the Gegenaobao formation is similar with the Izu–Bonin–Mariana arc, relating to subduction initiation. Based on the characteristics of exposed rocks and zircon U-Pb ages of andesite and granitoid rocks in the study area, we conclude the onset subduction of Mongol-Okhotsk Plate beneath the Erguna massif may occur at early-middle Permian.     

The origin of the Naturaliste Plateau,SE Indian Ocean: Implications from dredged basalts

Although a 1972 dredging by USNS Eltanin from the submarine Naturaliste Plateau was reported to yield rocks of continental origin, a re‐examination of the dredge haul shows that the rock clasts are in fact altered tholeiitic basalts. They have affinities both with MOR basalts and, especially, within‐plate basalts. Petrographically they correlate most closely with the Bunbury Tholeiitic Suite on the Australian mainland to the east. The basalts are reworked cobbles in a manganiferous Quaternary slump mass, which contains a quartz‐rich, felsic, detrital mineral suite with a granite‐gneiss provenance. The basalt cobbles were part of a basal conglomerate, which covered large areas of the Plateau. It is suggested that this was laid down from nearby elevated volcanic structures formed during the inception of seafloor spreading and the separation of Greater India from Australia at about 122 Ma BP.     

华北板块北缘中段二叠纪的构造属性:来自火山岩锆石U-Pb年代学与地球化学的制约

为了探究苏吉组基性岩岩浆的起源和演化,对华北板块北缘中段土牧尔台地区苏吉组玄武安山岩进行了锆石LA-ICP-MS U-Pb定年和岩石地球化学研究,并用此来制约该地区的构造演化历史。苏吉组玄武安山岩中的锆石形态较好,均为自形—半自形,具有明显的震荡环带,且Th/U值为0.61~1.06,反映其为典型的岩浆成因。测年结果显示,苏吉组玄武安山岩形成于早二叠世((279.0±2.1)Ma)。地球化学特征表明,苏吉组玄武安山岩贫硅(49.0%~55.2%)和HFSEs,富铝(15.70%~17.80%)、镁(Mg#=49.84~67.02)和LILEs,具有较低的稀土元素总量和较平缓的稀土分配方式,显示了岛弧火山岩的地球化学特征,加之本组安山岩具有高镁安山岩的属性,所以该组火山岩源区应为受俯冲板片流体交代的亏损地幔楔。综上,笔者认为苏吉组安山岩形成于俯冲的构造背景下,在早二叠世古亚洲洋未闭合。     

Geochemistry and petrogenesis of Permian granitoids in the northwestern margin of the North China Craton: insights from the Dongshengmiao pluton,Inner Mongolia

Permian granitoid emplacement represents one of the most important tectonothermal events in the northern margin of the North China Craton (NCC). In this study, we collected geochronological and geochemical data of the regional Permian granitoid in the northwestern margin of the NCC, and investigated the Dongshengmiao pluton, using it as an example to constrain the regional granitoid petrogenesis and its geodynamic settings. The Dongshengmiao pluton contains porphyritic granite and quartz diorite. LA-ICP-MS zircon U-Pb dating results have constrained the granitoid emplacement to be ca. 287?275 Ma. The Dongshengmiao granitoids have a SiO₂ range of 58.4?76.5%, moderate to high alkali content (Na₂O + K₂O = 5.16–7.94%), and are rich in large-ion lithophile elements (LILEs; e.g. Rb, Ba) and depleted in high-field strength elements (HFSEs; e.g. Nb, Ta, Ti). The zircons in quartz diorite have ε_Hf(t) values of ?15.6 to ?11.1 with two-stage Hf model ages (T_DM2) of 1997–2281 Ma, suggesting that the magma was derived from partial melting of old continental materials. In contrast, porphyritic granite shows variable Hf isotopic composition with ε_Hf(t) values of ?13.7 to ?2.6 and T_DM2 of 1471–2167 Ma, indicating a heterogeneous magma source. Besides the Dongshengmiao pluton, all the Permian granitoids in the northwestern margin of the NCC exhibit similar geochemical characteristics, including enrichment in LILEs, depletion in Nb and Ta, and enriched Hf isotopic signatures. The comprehensive geochemical data indicate that these Permian granitoids are derived from magma mixing between dominant partial melting of ancient felsic crustal materials and minor juvenile basaltic magma. Tectonically, the Dongshengmiao and other granitoids in the northwestern margin of the NCC may have been formed in a post-collisional extensional setting.     

Middle Permian high Sr/Y monzogranites in central Inner Mongolia: reworking of the juvenile lower crust of Bainaimiao arc belt during slab break-off of the Palaeo-Asian oceanic lithosphere

ABSTRACT

The high Sr/Y geochemical feature of granitoids can be attributed to various mechanisms, and elucidating genesis of high Sr/Y granitoids provides insights into the material recycling and magmatic processes at depth. In southeastern Central Asian Orogenic Belt (CAOB), many Middle Permian granitoids exhibit high Sr/Y ratios, but their origins remain unclear, inhibiting a comprehensive understanding of the magmatic response to the final closure of the Palaeo-Asian ocean. Here we present new zircon U-Pb ages, Lu-Hf isotopes and whole-rock geochemical data for the Middle Permian high Sr/Y monzogranites from central Inner Mongolia, southeastern CAOB. LA-ICP-MS zircon U-Pb data shows that these high Sr/Y rocks were emplaced during 273–261 Ma. They are calc-alkaline, sodium-rich and metaluminous to weakly peraluminous, with enriched large-ion lithophile elements (Rb, Th, K and Pb) and depleted high field strength elements (Nb, Ta, P and Ti), suggesting a mafic lower crustal source rather than evolved potassic crustal materials. Their relatively low (Gd/Yb)_N (1.1–2.0), (Dy/Yb)_N (1.0–1.3), Nb/Ta (7.9–10.9) ratios and flat heavy rare earth element patterns are characteristics of derivation from a relatively shallow depth with amphibolite as dominant residue. They also have highly variable ε_Hf(t) values (?8.2 to +10.0) and T_DMC (1814 to 649 Ma), similar to those of the Early Palaeozoic high Sr/Y intrusions along the Bainaimiao arc belt. Combined with data from literatures, we suggest that the high Sr/Y monzogranites in this study were probably generated by reworking of the newly underplated juvenile high Sr/Y lower crust of the Bainaimiao arc belt. Moreover, taking into account the regional investigations, the sublinear distributed Middle Permian magmatic rocks in the southeastern CAOB were likely associated with the incipient slab break-off of the Palaeo-Asian oceanic lithosphere following initial collision between the North China craton and the South Mongolia terranes.     

The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite

Structural and fabric analysis of the well-exposed Hilti mantlesection, Oman ophiolite, suggests that shear zone development,which may have resulted from oceanic plate fragmentation, wasinfluenced by pre-existing mantle fabric present at the paleo-ridge.Detailed structural mapping in the mantle section revealed agently undulating structure with an east–west flow direction.A NW–SE strike-slip shear zone cuts across this horizontalstructure. The crystal preferred orientation (CPO) of olivinewithin the foliation is dominated by (010) axial patterns ratherthan more commonly observed (010)[100] patterns, suggestingthat the horizontal flow close to the Moho involved non-coaxialflow. Olivine CPO within the shear zone formed at low temperatureis characterized by (001)[100] patterns and a sinistral senseof shear. The olivine CPO becomes weaker with progressive mylonitizationand accompanying grain size reduction, and ultimately developsinto an ultra-mylonite with a random CPO pattern. The olivine[010]-axis is consistently sub-vertical, even where the horizontalfoliation has been rotated to a sub-vertical orientation withinthe shear zone. These observations suggest that the primarymechanical anisotropy (mantle fabric) has been readily transformedinto a secondary structure (shear zone) with minimum modification.This occurred as a result of a change of the olivine slip systemsduring oceanic detachment and related tectonics during cooling.We propose that primary olivine CPO fabrics may play a significantrole in the subsequent structural development of the mantle.Thus, the structural behavior of oceanic mantle lithosphereduring subduction and obduction may be strongly influenced byinitial mechanical anisotropy developed at an oceanic spreadingcenter. KEY WORDS: mantle lithosphere; anisotropy; shear zone; olivine CPO; Oman ophiolite     

长江中下游地区蝌蚪山晚中生代玄武岩的地球化学研究: 岩石圈地幔性质与演化的制约

长江中下游地区繁昌火山盆地蝌蚪山晚白垩世玄武岩为硅饱和岩石, SiO2含量在 47.63%～ 50.02%之间,在 TAS图上位于碱性和亚碱性的分界线上,多数属于玄武岩,少数为粗面玄武岩. MgO含量较低( 3.72%～ 5.58%),但 Mg#值较高( 61～ 71).岩石富集大离子亲石元素 Ba、 Th、 U、 LREE和 Pb,亏损高场强元素 Nb、 Zr和 Ti,具富集的 Sr、 Nd和 Pb同位素组成.初始 87Sr/86Sr(t)比值介于 0.706 5～ 0.706 6之间,ε Nd(t)值介于-5.5～-7.3之间.在 87Sr/86Sr(t)-ε Nd(t)相关图上投在富集的第Ⅳ象限,并趋向于 EMⅡ地幔端员. 206Pb/204Pb(t)、 207Pb/204Pb(t)和 208Pb/204Pb(t)比值分别为 17.928～ 18.311、 15.426～ 15.621和 37.785～ 38.525,在 Pb同位素相关图上处在 DMM和 EMⅡ地幔端员之间.蝌蚪山玄武岩的地球化学特征表明其原始岩浆来源于岩石圈地幔,底侵到壳幔边界后曾有一段时间的滞留,并经历了一定程度的以橄榄石和斜方辉石为主的结晶分异,随后在上升过程中没有受到明显的地壳物质的混染.本地区晚中生代时期陆下岩石圈地幔具有同位素富集的性质,曾受到过古老俯冲事件中析出流体 /熔体的交代.与本地区新生代玄武岩地球化学性质的对比表明,从晚中生代至新生代,由于岩石圈拉张和软流圈的上涌,长江中下游地区发生了岩石圈的减薄,其过程和华北地块东部的岩石圈减薄事件基本一致.造成华北地块东部和华南地块东部的岩石圈减薄具有一致的动力学机制和背景,很可能是晚中生代时期古太平洋板块向亚洲大陆之下俯冲造成的弧后盆地的拉张减薄.     

The Xiong'er volcanic belt at the southern margin of the North China Craton: Petrographic and geochemical evidence for its outboard position in the Paleo-Mesoproterozoic Columbia Supercontinent

The Xiong'er volcanic belt, covering an area of more than 60,000 km² along the southern margin of the North China Craton, has long been considered an intra-continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the Paleo-Mesoproterozoic supercontinent Columbia. However, such interpretations cannot be accommodated by lithology, mineralogy, geochemistry and geochronology of the volcanic rocks in the belt. Lithologically, the Xiong'er volcanic belt is dominated by basaltic andesite and andesite, with minor dacite and rhyolite, different from rock associations related to continental rifts or mantle plumes, which are generally bimodal and dominated by mafic components. However, they are remarkably similar to those rock associations in modern continental margin arcs. In some of the basaltic andesites and andesites, amphibole is a common phenocryst phase, suggesting the involvement of H₂O-rich fluids in the petrogenesis of the Xiong'er volcanic rocks. Geochemically, the Xiong'er volcanic rocks fall in the calc-alkaline series, and in most tectono-magmatic discrimination diagrams, the majority of the Xiong'er volcanic rocks show affinities to magmatic arcs. In the primitive mantle normalized trace-element diagrams, the Xiong'er volcanic rocks show enrichments in the LILE and LREE, and negative Nb–Ta–Ti anomalies, similar to arc-related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge. Nd-isotope compositions of the Xiong'er volcanic rocks suggest that 5–15% older crust has been transferred into the upper lithospheric mantle by subduction-related recycling during Archean to Paleoproterozoic time. Available SHRIMP and LA-ICP-MS U–Pb zircon age data indicate that the Xiong'er volcanic rocks erupted intermittently over a protracted interval from 1.78 Ga, through 1.76–1.75 Ga and 1.65 Ga, to 1.45 Ga, though the major phase of the volcanism occurred at 1.78–1.75 Ga. Such multiple and intermittent volcanism is inconsistent with a mantle plume-driven rifting event, but is not uncommon in ancient and existing continental margin arcs. Taken together, the Xiong'er volcanic belt was most likely a Paleo-Mesoproterozoic continental magmatic arc that formed at the southern margin of the North China Craton. Similar Paleo-Mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of Laurentia, the southern margin of Baltica, the northwestern margin of Amonzonia, and the southern and eastern margins of the North Australia Craton, which are considered to represent subduction-related episodic outbuilding on the continental margins of the Paleo-Mesoproterozoic supercontinent Columbia. Therefore, in any configuration of the supercontinent Columbia, the southern margin of the North China Craton could not have been connected to any other continental block as proposed in a recent configuration, but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the North China Craton.