Mesozoic and early Cenozoic tectonic convergence-to-rifting transition prior to opening of the South China Sea

We have investigated Mesozoic geological problems around the South China Sea (SCS) based on gravimetric, magnetic, seismic, and lithofacies data. Three-dimensional analytical signal amplitudes (ASA) of magnetic anomalies clearly define the inland tectonic boundaries and the residual Mesozoic basins offshore. The ASA suggest that the degree of magmatism and/or the average magnetic susceptibility of igneous rocks increase southeastwards and that late-stage A-type igneous rocks present along the coast of southeast China possess the highest effective susceptibility. The geophysical data define Mesozoic sedimentary and tectonic structures and reveal four major unconformities [Pz/T–J, T–J/J, J/K, and Mesozoic/Cenozoic (Pz, Palaeozic; T, Triassic; J, Jurassic; K, Cretaceous)], corresponding to regional tectonic events revealed by nine palaeogeographic time slices based on prior geological surveys and our new fieldwork. Showing both sedimentary and volcanic facies and regional faults, our palaeogeographic maps confirm an early Mesozoic northwestward-migrating orogeny that gradually obliterated the Tethyan regime, and a middle-to-late Mesozoic southeastward migration and younging in synchronized extension, faulting, and magmatism. Three major phases of marine deposition developed but were subsequently terminated by tectonic compression, uplift, erosion, faulting, rifting, and/or magmatism. The tectonic transition from the Tethyan to Pacific regimes was completed by the end of the Middle Triassic (ca. 220 Ma), reflecting widespread Mesozoic orogeny. The transition from an active to a passive continental margin occurred at the end of the Early Cretaceous (ca. 100 Ma); this was accompanied by significant changes in sedimentary environments, due likely to an eastward retreat of the palaeo-Pacific subduction zone and/or to the collision of the West Philippine block with Eurasia. The overall Mesozoic evolution of southeast China comprised almost an entire cycle of orogenic build-up, peneplanation, and later extension, all under the influence of the subducting palaeo-Pacific plate. Continental margin extension and rifting continued into the early Cenozoic, eventually triggering the Oligocene opening of the SCS.     

Avulsion regimes in southeast Texas rivers

Avulsions – relatively sudden changes in course, or establishment of new anabranches – are an important process in alluvial rivers. Their key role in floodplain construction and alluvial architecture, and the general conditions favouring avulsions, are well known. However, avulsion processes and evolution, and the factors controlling avulsion regimes, are poorly understood. In the southeast Texas coastal plain, where avulsions are common features of the river valleys, avulsions were studied on the lower Brazos, Navasota, Trinity, Neches and Sabine rivers using a combination of aerial imagery, digital elevation models and field surveys. Avulsions have important influences on the surface morphology and contemporary processes in all five rivers. Features associated with avulsions are active and distinct throughout the study area, and all the rivers have experienced geologically (if not historically) recent avulsions. However, no two of the study rivers have the same contemporary avulsion regime. First‐order differences in avulsion style are controlled by the stage of valley filling, and within the three rivers characterized by an unfilled incised valley, antecedent morphology associated with late Quaternary and Holocene coastal and fluvial‐deltaic processes accounts for the major differences. In the Navasota (27 avulsions in 185 km) and Neches (21 in 340 km) rivers, subchannels associated with avulsions exist in all stages of development from active to infilled, and some have occurred in recent decades. The other rivers have fewer avulsions, but both the Sabine and Trinity have experienced historic channel shifts. Only the Brazos River has experienced no avulsions within the past c. 300 years. Results show that even within a region of similar environmental controls and geological history local variations in inherited morphology can result in different avulsion regimes. Copyright © 2008 John Wiley & Sons, Ltd.     

First record of contourites from Lower Devonian Liptrap Formation in southeast Australia

Typical contourite deposits associated with submarine turbidite fan deposits are recognized for the first time from the Lower Devonian Liptrap Formation at Cape Liatrap, Victoria in southeast Australia. The contourites are well integrated within the turbidite fan deposits and are characterized by thin (5-8 cm), lenticular, well-sorted coarse-grained siltstones to fine-grained sandstones with current-ripples and cross beddings. The palaeocurrent directions of the turbidite fan and contourites are perpendicular to each other, with the former directed generally westward while the latter varying from 165° to 190° southward. In view of the facies types and architecture, we suggest that the turbidite fan was developed at the base of a westward inclined palaeo-slope, at the front of which the contourites were deposited as a result of southward flowing deep-sea contour (geostrophic) currents. The depositional setting inter- preted for the Liptrap Formation thus may provide a provisional model for the Lower Devonian conti- nental slope and abyssal basin environment in the southeastern part of the Melbourne Trough.     

Newly-generated Daliangshan Fault Zone—Shortcutting on the central section of Xianshuihe-Xiaojiang fault system

The Daliangshan fault zone is the eastern branch in the central section of Xianshuihe-Xiaojiang fault system. It has been neglected for a long time, partly because of no destructive earthquake records along this fault zone. On the other hand, it is located on the remote and inaccessible plateau. So far it was excluded as part of the Xianshuihe-Xiaojiang fault system. Based on the interpretation of aerophotographs and field investigations, we document this fault zone in detail, and give an estimation of strike-slip rate about 3 mm/a in Late Quaternary together with age dating data. The results suggest that the Daliangshan fault zone is a newly-generated fault zone resulted from shortcutting in the central section of Xianshuihe-Xiaojiang fault system because of the clockwise rotation of the Southeastern Tibetan Crustal Block, which is bounded by the Xianshuihe-Xiaojiang fault system. Moreover, the shortcutting may make the Daliangshan fault zone replace the Anninghe and Zemuhe fault zones gradually, and finally, the later two fault zones will probably die out with the continuous clockwise rotation.     

假等称笔石(Pseudisograptus)在桂东南的发现

1996— 1 997年间笔者曾多次鉴定广西区域调查研究院送来的化石标本 ,其中有些化石采自广西东南边陲的博白县龙潭镇以东的六角田 ,产出于曾被归属志留系的一套轻变质的灰黑色泥岩中 ,因受加里东期花岗岩体侵入的影响 ,所送标本受到轻度变质的作用 ,保存极差 ,且数量不足。化石丰度不高 ,其中主要是一些对笔石类断枝 ,能作确切鉴定的只有一种假等称笔石即剑柄假等称笔石哈氏亚种( Pseudisograptus mamubriatus harris Cooper &Ni)。假等称笔石属地质历程短 ,在澳大利亚相当发育 ,产出层位为 Castlemainian阶上部至 Yapeenian阶 ,大致可与英…     

海相沉积成因的黑滑石矿床及其找矿方向

海相沉积成因的黑滑石矿床是近年来在我国发现的新的成因类型矿床，本文除着重介绍在川东南发现的海相沉积成因的黑滑石矿床地质特征和矿床成因外，还引进非金属矿床含矿建造和成矿系列的概念，提示黑滑石矿及与其共生伴生的其它非金属矿的找矿方向。     

吉东南新太古代晚期片麻岩类的时代、成因及其对早期地壳形成演化的制约

吉林省东南部通化地区广泛出露早前寒武纪花岗质片麻岩类,其岩石组成、形成时代和成因对深化认识华北克拉通东北缘早期地壳形成演化历史具有重要意义。本文系统的岩石学、锆石U-Pb年代学、元素和Lu-Hf同位素地球化学等研究结果表明,这些花岗质片麻岩类按照SiO₂质量分数总体可划分为高硅和低硅两组：前者主要由二长花岗质片麻岩、奥长花岗质片麻岩及英云闪长质片麻岩组成,其原岩形成于2 549~2 557 Ma;而后者由石英二长闪长质片麻岩及花岗闪长岩质片麻岩组成,其原岩形成于2 534~2 552 Ma;并且两组岩石都含有约2 600 Ma的捕获锆石,共同遭受了约2 500 Ma变质作用的影响。地球化学分析结果显示,低硅岩组具有较高的MgO、CaO、Na₂O质量分数,属于高钾—中钾钙碱性系列,并且富集LREE、亏损HREE和HFSE;与之相比,高硅岩组则具有较低的MgO和CaO质量分数,显示更强烈的轻、重稀土元素分馏以及Nb、Ta、P、Ti等亏损的特征;但两者均具有较弱的正或负Eu异常。结合区域最新研究成果,认为研究区低硅和高硅两组岩石应具有相同的源区,其形成可能与大洋板片俯冲、岩浆底侵引起的地壳部分熔融作用有关。此外,两组岩石具有相似的ε_Hf（t）值（2.72~7.95）和模式年龄（2.86~2.55 Ga）,暗示区域主要存在新太古代晚期地壳生长事件;结合区域内变质火山岩的研究进展,认为吉林省东南部通化地区花岗质片麻岩类可能形成于活动大陆边缘的构造背景。     

Two Periods of Skarn Mineralization in the Baizhangyan W–Mo Deposit,Southern Anhui Province,Southeast China: Evidence from Zircon U–Pb and Molybdenite Re–Os and Sulfur Isotope Data

The recently discovered Baizhangyan skarn‐porphyry type W–Mo deposit in southern Anhui Province in SE China occurs near the Middle–Lower Yangtze Valley polymetallic metallogenic belt. The deposit is closely temporally‐spatially associated with the Mesozoic Qingyang granitic complex composed of g ranodiorite, monzonitic g ranite, and alkaline g ranite. Orebodies of the deposit occur as horizons, veins, and lenses within the limestones of Sinian Lantian Formation contacting with buried fine‐grained granite, and diorite dykes. There are two types of W mineralization: major skarn W–Mo mineralization and minor granite‐hosted disseminated Mo mineralization. Among skarn mineralization, mineral assemblages and cross‐cutting relationships within both skarn ores and intrusions reveal two distinct periods of mineralization, i.e. the first W–Au period related to the intrusion of diorite dykes, and the subsequent W–Mo period related to the intrusion of the fine‐grained granite. In this paper, we report new zircon U–Pb and molybdenite Re–Os ages with the aim of constraining the relationships among the monzonitic granite, fine‐grained granite, diorite dykes, and W mineralization. Zircons of the monzonitic granite, the fine‐grained granite, and diorite dykes yield weighted mean U–Pb ages of 129.0 ± 1.2 Ma, 135.34 ± 0.92 Ma and 145.3 ± 1.7 Ma, respectively. Ten molybdenite Re–Os age determinations yield an isochron age of 136.9 ± 4.5 Ma and a weighted mean age of 135.0 ± 1.2 Ma. The molybdenites have δ³⁴S values of 3.6‰–6.6‰ and their Re contents ranging from 7.23 ppm to 15.23 ppm. A second group of two molybdenite samples yield ages of 143.8 ± 2.1 and 146.3 ± 2.0 Ma, containing Re concentrations of 50.5–50.9 ppm, and with δ³⁴S values of 1.6‰–4.8‰. The molybdenites from these two distinct groups of samples contain moderate concentrations of Re (7.23–50.48 ppm), suggesting that metals within the deposit have a mixed crust–mantle provenance. Field observation and new age and isotope data obtained in this study indicate that the first diorite dyke‐related skarn W–Au mineralization took place in the Early Cretaceous peaking at 143.0–146.3 Ma, and was associated with a mixed crust–mantle system. The second fine‐grained granite‐related skarn W–Mo mineralization took place a little later at 135.0–136.9 Ma, and was crust‐dominated. The fine‐grained granite was not formed by fractionation of the Qingyang monzonitic granite. This finding suggests that the first period of skarn W–Au mineralization in the Baizhangyan deposit resulted from interaction between basaltic magmas derived from the upper lithospheric mantle and crustal material at 143.0–146.3 and the subsequent period of W–Mo mineralization derived from the crust at 135.0–136.9 Ma.     

桂东南马山沿灵山断裂出露的印支期洋岛玄武岩及其区域构造意义

桂东南沿灵山断裂出露的马山印支期玄武岩的常量元素特征指示它属洋岛碱性玄武岩,微量元素含量显示出大洋板内玄武岩的特点。稀土元素配分型式表现为一组近于平行的向右缓倾曲线,(La/Yb)N=11.69,(Ce/Yb)N=8.64,(Ce/Sm)N=2.41,稀土总量随全碱含量增大有升高的趋势,表明它是亚速尔型洋岛。这首次为桂东南及毗邻的粤西南地区属古特提斯造山带、灵山断裂是古特提斯分支洋盆闭合后的缝合线提供了来自洋盆内地质记录的证据。结合粤桂交界区的最近研究成果提出了研究灵山洋演化的工作模式。     

桂东南荔枝生长营养元素农业地球化学研究

桂东南是中国重要的荔枝产区,以梧州地区为例,重点对沙头和古凤两处荔枝种植园进行了生态地球化学调查,采集了荔枝园施肥圈内、外土壤,出露的代表性基岩,以及荔枝叶、青果和熟果样品,实测了大量和微量矿质元素、稀土元素,以及土壤营养元素有效态的含量。通过计算荔枝对土壤营养元素的吸收率及荔枝生长与结果期对营养元素的需求,查明本区荔枝生长的特征营养元素为Ca、K、Mg、Mn、P、Zn、Cu、B、Fe和REE。荔枝结果期的特需营养元素为P、Mg、Mo、Cu、Zn、Ko荔枝结果早和晚期特需元素分别为Ca、Si、B和Cu、Fe、K。在岩石-土壤-荔枝株生态循环系统中,本区营养元素在土壤中保持一定的积累,提供荔枝生长的需求,因此为荔枝宜种区;但全区土壤B缺乏,Zn、Mo局部不足。Ca、K、Zn在两区岩石土壤中分布不同,可能是造成沙头荔枝比古凤荔枝迟熟的主要农业地球化学因素。研究区有害元素Pb、Cd在土壤中积累是值得关注的土壤环境问题。