Interpretation of neovolcanic versus palaeovolcanic sand grains: an example from Miocene deep-marine sandstone of the Topanga Group (Southern California)

Despite abundant data on volcaniclastic sand(stone), the compositional, spatial and temporal distribution of volcanic detritus within the sedimentary record is poorly documented. One of the most intricate tasks in optical analysis of sand(stone) containing volcanic particles is to distinguish grains derived by erosion of ancient volcanic rocks (i.e. palaeovolcanic, noncoeval grains) from grains generated by active volcanism (subaqueous and/or subaerial) during sedimentation (neovolcanic, coeval grains). Deep-marine volcaniclastic sandstones of the Middle Topanga Group of southern California are interstratified with 3000-m-thick volcanic deposits (both subaqueous and subaerial lava and pyroclastic rocks, ranging from basalt, andesite to dacite). These rocks overlie quartzofeldspathic sandstones (petrofacies 1) of the Lower Topanga Group, derived from deep erosion of a Mesozoic magmatic arc. Changes in sandstone composition in the Middle Topanga Group provide an example of the influence of coeval volcanism on deep-marine sedimentation. Volcaniclastic strata were deposited in deep-marine portions of a turbidite complex (volcaniclastic apron) built onto a succession of intrabasinal lava flows and on the steep flanks of subaerially emplaced lava flows and pyroclastic rocks. The Middle Topanga Group sandstones are vertically organized into four distinctive petrofacies (2–5). Directly overlying basalt and basaltic-andesite lava flows, petrofacies 2 is a pure volcanolithic sandstone, including vitric, microlitic and lathwork volcanic grains, and neovolcanic crystals (plagioclase, pyroxene and olivine). The abundance of quenched glass (palagonite) fragments suggests a subaqueous neovolcanic provenance, whereas sandstones including andesite and minor basalt grains suggest subaerial neovolcanic provenance. This petrofacies probably was deposited during syneruptive Periods, testifying to provenance from both intrabasinal and extrabasinal volcanic events. Deposited during intereruptive periods, impure volcanolithic petrofacies 3 includes both neovolcanic (85%) and older detritus derived from plutonic, metamorphic and palaeovolcanic rocks. During post-eruptive periods, the overlying quartzofeldspathic petrofacies 4 and 5 testify to progressive decrease of neovolcanic detritus (48–14%) and increase of plutonic-metamorphic and palaeovolcanic detritus. The Upper Topanga Group (Calabasas Formation), conformably overlying the Middle unit, has dominantly plutoniclastic sandstone (petrofacies 6). Neovolcanic detritus is drastically reduced (4%) whereas palaeovolcanic detritus is similar to percentages of the Lower Topanga Group (petrofacies 1). In general, the volcaniclastic contribution represents a well-defined marker in the sedimentary record. Detailed compositional study of volcaniclastic strata and volcanic particles (including both compositional and textural attributes) provides important constraints on deciphering spatial (extrabasinal vs. intrabasinal) and temporal relationships between neovolcanic events (pre-, syn-, inter- and post-eruptive periods) and older detritus.     

Unravelling provenance from Eocene–Oligocene sandstones of the Thrace Basin,North‐east Greece

New sandstone petrology and petrostratigraphy provide insights on Palaeogene (Middle Eocene to Oligocene) clastics of the Thrace Basin in Greece, which developed synchronously with post‐Cretaceous collision and subsequent Tertiary extension. Sandstone petrofacies are used as a tool to unravel complex geodynamic changes that occurred at the southern continental margin of the European plate, identifying detrital signals of the accretionary processes of the Rhodope orogen, as well as subsequent partitioning related to extension of the Rhodope area, followed by Oligocene to present Aegean extension and wide magmatic activity starting during the Early Oligocene. Sandstone detrital modes include three distinctive petrofacies: quartzolithic, quartzofeldspathic and feldspatholithic. Major contributions are from metamorphic basement units, represented mostly by low to medium‐grade lithic fragments for the quartzolithic petrofacies and high‐grade metamorphic rock fragments for the quartzofeldspathic petrofacies. Volcaniclastic sandstones were derived from different volcanic areas, with a composition varying from dominantly silicic to subordinate intermediate products (mainly rhyolitic glass, spherulites and felsitic lithics). Evolution of detrital modes documents contributions from three key source areas corresponding to the two main crystalline tectonic units: (i) the Variegated Complex (ultramafic complex), in the initial stage of accretion (quartzolithic petrofacies); (ii) the Gneiss–Migmatite Complex (quartzofeldspathic petrofacies); and (iii) the Circum‐Rhodope Belt. The volcaniclastic petrofacies is interbedded with quartzofeldspathic petrofacies, reflecting superposition of active volcanic activity on regional erosion. The three key petrofacies reflect complex provenance from different tectonic settings, from collisional orogenic terranes to local basement uplift and volcanic activity. The composition and stratigraphic relations of sandstones derived from erosion of the Rhodope orogenic belt and superposed magmatism after the extensional phase in northern Greece provide constraints for palaeogeographic and palaeotectonic models of the Eocene to Oligocene western portions of the Thrace Basin. Clastic detritus in the following sedimentary assemblages was derived mainly from provenance terranes of the Palaeozoic section within the strongly deformed Rhodope Massif of northern Greece and south‐east Bulgaria, from the epimetamorphic units of the Circum‐Rhodope Belt and from superposed Late Eocene to Early Oligocene magmatism related to orogenic collapse of the Rhodope orogen. The sedimentary provenance of the Rhodope Palaeogene sandstones documents the changing nature of this orogenic belt through time, and may contribute to a general understanding of similar geodynamic settings.     

A framework for the palaeozoic geology of the southern part of the New England Geosyncline

The southern part of the New England Geosyncline is divided by the Peel Fault into two major zones, termed the Tablelands Complex to the east and the Tamworth Belt to the west. Because of stratigraphical similarities the Hastings Block is correlated with the Tamworth Belt. Seven major lithostratigraphic associations are recognized in the Tablelands Complex. The Woolomin Association is a deepwater marine chert‐jasper‐basic volcanic‐dominated sequence in which sandstones are rare. The Sandon Association is a turbidite sequence in which minor chert, jasper, and basic volcanics are present. The Coffs Harbour Association is a deepwater marine turbidite sequence and is distinguished from the Sandon Association by the absence of chert, jasper, and basic volcanics. The Nambucca Association is also deepwater marine in nature and is dominated by diamictites deposited by mass movements. The Silverwood Association contains components of a volcanic arc, and the Dummy Creek Association consists of terrestrial to shallow‐water marine sediments deposited in rim synclines associated with the granitic intrusions. The above associations contain sandstones which are predominantly quartz‐poor lithic greywackes and these differ from the Beenleigh Association which contains continental shelf to deepwater marine deposits in which the sandstones are quartz‐rich. Recognition of the associations has been hindered by the intrusion of four granitic suites, metamorphism of the sediments under differing conditions of temperature and pressure, and fragmentation by several large‐scale faults into numerous blocks.     

Evolution of a provenance as revealed by petrographic analyses of Cretaceous formations in the Queen Charlotte Islands, British Columbia, Canada

Two contrasting marine sedimentary facies, the Haida Formation of sandy and argillaceous sediments and the conformably overlying Honna Formation of gravelly sediments were formed within different types of tectonic basins during mid- to Late Cretaceous time. The sediments of both formations were derived from the east. Sandstones from the two formations show characteristics of mature magmatic arc provenance and are classified as lithic and feldspathic arenites. However, the Honna sandstones are more feldspathic and less quartzose than the Haida sandstones. The Honna sandstones have many volcanic rock fragments (VRF) but the Haida sandstones do not. Feldspars of the VRF-rich Honna sandstones, however, do not seem to have been derived from the breakdown of volcanic rocks. The observed petrographic differences between the two formations can be ascribed to a process in which the volcanic cover and the basement rock denudation took place simultaneously in the source area during deposition of the Honna Formation.     

Petrographic variations within thick turbidite sequences: an example from the late Palaeozoic of eastern Australia

The subdivision of thick sequences of turbidite sediments has been problematical because of the monotonous nature of the units. One method, of using detailed detrital petrography for a large number of specimens, has delineated variations with a sequence of Late Palaeozoic age in eastern Australia. The rocks occur within a single structural block and are all members of one sedimentary petrographic province. They have been subdivided into three stratigraphic units (Moombil Beds, Brooklana Beds and Coramba Beds) and greywackes from these units are quartz-poor to quartz-intermediate, feldspathic or volcanolithic types. Dacitic volcanism has provided most of the detritus and the contribution from non-volcanic sources is small. The Coramba Beds are further subdivided into four petrographic units which are parallel to the stratigraphic boundaries. These lithostratigraphic units are based on the presence or absence of detrital hornblende, and the relative ratio of volcanic lithic fragments to feldspar. Vertical petrographic variations within the entire sequence indicate that although the acid volcanic source was predominant throughout the time of deposition, there is a noticeable increase in the contribution from intermediate-volcanic, acid-plutonic, low-grade metamorphic and sedimentary sources towards the top of the sequence. Detrital hornblende is also present in the upper parts of the sequence.     

Longitudinal petrographic variations in a Middle Ordovician trench deposit, Central Appalachian orogen

Petrographic analysis of Middle Ordovician turbidite sandstones of the Greenwich slice of the Hamburg klippe (eastern Pennsylvania), inferred to be part of a fossil subduction complex, define three coeval petrofacies. The Jonestown petrofacies was derived from felsic plutonic and less abundant metasedimentary rocks, whereas the Windsor Township, the most extensive petrofacies, and Werleys Corner petrofacies were derived from sources characterized by various proportions of sedimentary/metasedimentary, plutonic, and volcanic rocks. The presence of minor but conspicuous extrabasinal carbonate and microlitic volcanic lithic fragments together with higher percentages of polycrystalline quartz, serve to distinguish the Werleys Corner from the Windsor Township petrofacies. It is conceivable that sandstones of the Greenwich slice were derived from microplates inferred to have existed to the southeast of the proto-North American plate in Early Palaeozoic time. The variations in sandstone composition along the length of the Greenwich slice may be explained by post-accretion tectonic juxtaposition of petrofacies derived from various sources. An equally plausible explanation involves transverse infilling of a channelized longitudinal transport system (Windsor Township petrofacies) by sediment derived from compositionally diverse source terranes orthogonal to the trench (Jonestown and Werleys Corner petrofacies).     

Unraveling multiple provenance areas using sandstone petrofacies and geochemistry: An example in the southern flank of the Golfo San Jorge Basin (Patagonia,Argentina)

The aim of this paper is to study the provenance of Late Cretaceous sandstones deposited along the south flank of the Golfo San Jorge Basin. For this purpose, detrital modes of three hundred thirty-seven sandstone samples collected in the Mina del Carmen, Bajo Barreal, and Cañadón Seco Formations were studied in ten oil fields. According to the modal composition of the sandstones, six petrofacies were defined allowing the identification of not only principal, but also secondary provenance areas. The QVM and VQM petrofacies are more than 20% metamorphic, sedimentary, and polycrystalline quartz clasts (Lm + Ls + Qpg > 20%), evidencing a secondary signal of basement supply masked by a predominant volcanic provenance. The petrofacies VP and VF are characterized by Lm + Ls + Qpg <20% and more than 20% total feldspar (Pm + Om >20%.), which indicate a supply of sediment from volcanic terrains and scarce derivation of materials from basement rocks. Based on the plagioclase/k-feldspar ratio, the VF petrofacies is interpreted to be dominated by the supply of sand grains from the Andean volcanic-arc, while VP is supposed have originated through the erosion of intermediate volcanic rock outcroppings in the Macizo del Deseado. Finally, both the VQ and QV petrofacies show Lm + Ls + Qpg <20% and Pm + Om<20%, indicating a provenance of volcanic areas coupled with minor contributions from basement rocks. During the Late Cretaceous, the Golfo San Jorge Basin underwent a sag phase that was characterized by very scarce volcanism and tectonic activity. Although these conditions did not favor defined patterns in the vertical stacking of petrofacies, the sandstones exhibit remarkable changes in their regional distribution, which were determined by the paleogeography of the basin and differences in basement composition within the source areas. Finally, a paleogeographic model for sediment circulation in the basin is proposed. This model recognizes the main fluvial dispersal trends that flowed northwest to southeast and transported large amounts of volcanic clasts (associated with petrofacies VF-VQ). To the extent that rivers flowed eastward, a secondary supply from the Precambrian basement, which were composed of low-to high-grade metamorphic rocks, was also important (petrofacies association VQM and QVM). The southwestern area of the basin is dominated by VP petrofacies that record the supply of plagioclase-rich volcanic clasts. This petrofacies likely corresponds to the erosion of Jurassic volcanic units that crop out in the Macizo del Deseado.     

新疆东准噶尔喀姆斯特晚古生代沉积记录:物源和沉积作用研究

东准噶尔喀姆斯特下泥盆统阿拉比也巴斯他乌组和下石炭统卡姆斯特组代表陆壳增生不同阶段的沉积响应.碎屑岩碎屑组成模式和地球化学分析结果表明阿拉比也巴斯他乌组形成于大洋-活动大陆过渡型构造环境,物源区主要为发育在过渡型地壳之上的岩浆岛弧;卡姆斯特组形成于活动大陆型构造环境,物源区主要为大陆岛弧环境的切割岩浆弧.沉积相、相组合及生物生态等沉积特征显示两组的沉积环境分别为海底斜坡和海底扇中扇-外扇盆地平原.结合区域构造分析和地层对比研究,下泥盆统阿拉比也巴斯他乌组海底斜坡沉积是东准噶尔构造带早泥盆世弧后盆地沉积响应的主要记录,卡姆斯特组海底扇-海底平原沉积则主要记录了东准噶尔复合地体早石炭世晚期弧间残余海盆的沉积响应.两套沉积响应记录的环境演化受控于中亚型造山带复杂的造山作用.     

Applying sandstone petrofacies to unravel the Upper Carboniferous evolution of the Paganzo Basin, northwest Argentina

A compositional study of sandstones belonging to the lower section of the Paganzo Group (Middle Carboniferous–Early Permian) in the Paganzo Basin (northwestern Argentina) helps unravel the stratigraphic and paleogeographic evolution of the basin. Three morphotectonic units constitute the complex basement of the basin: (1) to the east, the igneous–metamorphic basement of the Sierras Pampeanas and Famatina systems; (2) to the west, the Precordillera, made up of Early and Middle Paleozoic sedimentary rocks; and (3) the Upper Paleozoic volcanic arc along the western boundary with the Río Blanco Basin. On the basis of sandstone detrital modes of the Lagares, Malanzán, Loma Larga, Guandacol, Tupe, Punta del Agua, and Río del Peñón formations, seven petrofacies are distinguished: quartzofeldespathic (QF), quartzofeldespathic-metamorphic enriched (QF-Lm), quartzofeldespathic-sedimentary enriched (QF-Ls), mixed quartzolithic (QL), quartzolithic-volcanic (QLv), volcanolithic-quartzose (LvQ), and volcanolithic (Lv). The spatial and temporal distribution of these petrofacies suggest an evolutive model for the Upper Paleozoic sedimentary filling of the basin that includes three “petrosomes”: (1) the basement petrosome, a clastic wedge of arkosic composition that diachronically prograded and thinned from east to west; (2) the recycled orogen petrosome, revealing the Protoprecordillera as a positive element in the western Paganzo Basin during the Namurian; and (3) the volcanic arc petrosome, recording volcanic activity along the western margin of Gondwana during the Westphalian.     

鄂尔多斯盆地陇东地区延长组砂岩储层岩石学特征

鄂尔多斯盆地陇东地区延长组是该地区的主要产油层,素以“低孔低渗”而闻名。根据岩心观察、薄片鉴定、粒度分析和扫描电镜、阴极发光和X 衍射等分析化验对该地区延长组储层的岩石学特征及其变化规律进行了深入研究。研究表明,陇东地区延长组砂岩储层岩石类型主要为细中粒岩屑质长石砂岩和长石质岩屑砂岩;成分成熟度较低;总体上从西部耿湾-环县向东至合水-塔儿湾,砂岩成分成熟度逐渐增高;下部的长8-长10及上部长1-长4+5较粗,为中细粒砂岩,中部长6、长7较细,多为粉-细砂岩。砂岩岩屑含量高、类型丰富,发育泥岩、白云岩等沉积岩岩屑,喷发岩、花岗岩等岩浆岩岩屑及板岩、千枚岩、片岩等变质岩岩屑,其中长8-长10以喷发岩岩屑为主,含量东西差异小,长1-长7以白云岩岩屑为主,含量西高东低;变质岩岩屑各层段都存在;云母在东部含量较高。按岩屑对成岩作用的影响分为刚性和塑性两大类,其中刚性可再细分为易溶与难溶两类,塑性可细分为易压实变形及易蚀变两类。此项研究成果对于分析延长期的盆地演化、物源类型及储层后期成岩作用及孔隙演化具有重要意义。     

Geochemistry of the Paleocene Clastic Rocks in Lishui Sag, East China Sea Shelf Basin: Implications for Tectonic Background and Provenance

The Lishui Sag, in the East China Sea Shelf Basin, is rich in hydrocarbons, with the major hydrocarbon-bearing layers being the Paleocene Mingyuefeng clastic rocks. Analysis of the implicit geologic background information of these Paleocene clastic rocks using petrological and geochemical methods has significant practical importance. These Paleocene sandstones are mainly lithic arenite, lithic arkose and greywacke, composed of K-feldspar, plagioclase, authigenic clays, silica and carbonates. As continental deposits, Yueguifeng clastic rocks have high aluminosilicate and mafic detritus contents, while the Lingfeng and Mingyuefeng Formations are rich in silica due to an oscillating coastal marine depositional environment. The major element contents of these Paleocene sandstones are low and have a concentrated distribution, indicating that the geochemical composition is non-epigenetic, transformed by sedimentary processes and diagenesis. The Yueguifeng detritus comprises recycled sediments, controlled by moderate weathering and erosion, while the Lingfeng and Mingyuefeng detritus is interpreted as primarily first-cycle materials due to low chemical weathering. In the Late Cretaceous to Early Paleocene, the Pacific Plate began subducting under the Eurasian Plate, causing an orogeny by plate collision and magma eruption due to the melting of subducted oceanic crust. This resulted in the dual tectonic settings of “active margin” and “continental island arc” in the East China Sea Shelf Basin. During the Late Paleocene, the Pacific Plate margin migrated eastward along with development of the Philippine Ocean Plate, and the tectonic setting of the Lishui Sag gradually turned into a passive continental margin. Detrital sources included both orogenic continental blocks and continental island arcs, and the parent rocks are primarily felsic volcanic rocks and granites.     

The arenig in South Wales: Sedimentary and volcanic processes during the initiation of a marginal basin

Marine clastic sediments and volcanics of Arenig (Ordovician) age crop out in South Wales. These were deposited after presumed late Tremadoc erosion and subsequent arc volcanicity. Arenig sedimentation was transgressive, and followed significant erosion of the arc volcanics. Arenig conglomerates, sandstones, and mudstones were deposited in deltaic and turbiditic systems. Storm and tidal processes influenced the shallow marine deposits. The minor rhyolitic volcanics extruded during the Arenig reflect the development of Ordovician marginal basin-type volcanics across Wales. Five sandstone petrofacies are defined and reflect differing proportions of these volcanics and of Cambrian and Precambrian basement material. Sedimentation patterns were controlled by intra-Arenig tectonism during an overall rise in sea level. Facies and petrofacies were ponded in small, interconnected, marine sub-basins. Earliest Arenig tectonism and sedimentation, also recognized in North Wales, reflects the initiation of a marginal basin in Wales.     

Sedimentary Facies,Provenance and Geochronology of the Heshangzhen Group: Implications for the Tectonic Evolution of the Eastern Jiangnan Orogen,South China

The Neoproterozoic Jiangnan orogen plays an important role in the study of the Precambrian tectonic evolution of South China. The tectonic nature of the Neoproterozoic sedimentary basins is still controversial, due to poor understanding of the sedimentary sequences and the lack of geochronological data. Here, we present sedimentological, provenance and geochronological data from the Heshangzhen Group in the eastern Jiangnan orogen. Sedimentological analysis shows that the Luojiamen Formation was deposited in a submarine fan, and the overlying Hongchicun Formation was deposited in front of a fan delta. The youngest detrital zircons constrain the lower Luojiamen and Hongchicun formations with ages of 827.3 ± 8.4 Ma and 825 ± 12 Ma, respectively. The sandstones of the Luojiamen Formation are characterized by a large number of intermediate to felsic volcanic grains, suggesting a volcanic arc source. In contrast, quartz and sedimentary lithic grains increase in the Hongchicun Formation, showing a new input from a collisional orogenic source. Detrital zircon from six sandstone samples in the Luojiamen and Hongchicun formations yield similar age spectra of 930–820 Ma with a peak at ca. 845–860 Ma, with one main cluster at 930–820 Ma. Detrital zircons of 930–845 Ma show a positive value of εHf(t)(+2.4 to +11, mean +7.6), which is similar to the volcanic arc of the nearby Shuangxiwu Group. There are a minor group of zircons with U-Pb ages ranging from 820 Ma to 845 Ma from the middle part of the Luojiamen Formation and Hongchicun Formation, with εHf(t) values between-20 to +2.4, which are consistent with the characteristics of the Shuangqiaoshan Group. within light of the bidirectional paleocurrents in the Luojiamen Formation, it is speculated that the zircons of 820–845 Ma were recycled from the Shuangqiaoshan Group, which is derived from a continental arc to the northwest. Our data suggests that the Luojiamen Formation was formed in an inter-arc basin, while the Hongchicun Formation was formed in an accretionary wedge-top basin. When juxtaposed with the conglomeratic characteristics at the bottom of the Luojiamen Formation, it is believed that the unconformity represented by the ‘Shen Gong Movement' reflects the rapid erosion and accumulation process of island arc volcanic material. The disconformity between the Luojiamen and Hongchicun formations is the imprint of transition from inter-arc basin to accretionary wedge-top basin,which represents the collision between the Shuangxiwu arc and the Yangtze Plate.     

Thermal Metamorphism in the trial harbour district,Tasmania

A Devonian granite complex intrudes Precambrian and Silurian siltstones and sandstones as well as (?) Cambrian volcanics and dunite.

Metamorphism of the Precambrian sediments is slight, and an andalusite‐bearing, pelitic hornfels is the only characteristic assemblage. The (?) Cambrian volcanics give rise to a variety of assemblages; (1) lime‐ and ferromagnesia‐rich (hypersthene — cummingtonite — labradorite; diopside — hornblende — labradorite); (2) magnesia‐rich (cordierite — hypersthene; cordierite — anthophyllite); (3) ultrabasic (olivine and/or pleonaste). Biotite (or phlogopite) is an almost invariable component, and garnet may also be present in these groups.

No significant metamorphism of the dunite is evident; minor development of veins and segregrations of aragonite, magnetite, phlogopite, brucite, chalcedony and antigorite may result from low‐grade hydrothermal activity Metamorphic assemblages in calcareous Silurian siltstones contain garnet, diopside, calcite and epidote.

A characteristic feature of the contact metamorphic aureole is the occurrence of diopside‐rich bodies in granite, volcanic hornfels, quartzite and dunite host rocks.     

秦岭造山带主要大地构造单元的新划分

根据近年来的地层、沉积、岩浆-火山和构造变形及岩石地球化学等方面研究新进展,结合前人的成果,按照大地构造相单元划分原则,将秦岭造山带分为13个主要构造单元: ①华北南缘陆坡带,包括第一层序的青白口系大庄组、震旦系罗圈组和寒武系,与之对应的豫西栾川群;第二层序的奥陶纪陶湾群;②北秦岭弧后杂岩带,以宽坪群和部分二郎坪群中的基性火山岩与碳酸盐岩的构造块体与变质的古生代深海碎屑岩混杂为特征;③秦岭岛弧杂岩带,由丹凤群不同的古洋隆块体、富水幔源岛弧基性岩浆杂岩、云架山群、斜峪关群和草滩沟群的岛弧钙碱性岩浆岩和火山岩及深海沉积物及秦岭群弧基底杂岩等构成,时间跨度为奥陶纪-石炭纪;④秦岭弧前盆地系,泥盆系及其它晚古生代地层是其主要充填物,同沉积断裂控制了一系列的次级盆地;⑤秦岭增生混杂带,由泥、砂岩组成的基质和基性、超基性岩、火山岩、灰岩、硅质岩等岩块构成,最终形成于二叠纪末-三叠纪初;⑥南秦岭岛弧杂岩带,碧口群的基性-中酸性火山岩和岩浆岩组成,称碧口弧;由三花石群的中基性火山岩以及西乡群的中酸性火山岩共同构成,称西乡弧;由耀岭河群和郧西群中基性熔岩和中酸性火山岩组成,称安康弧;⑦南秦岭弧前盆地系,碧口弧前盆地充填物是以碎屑岩为主的横丹群和关家沟群;西乡弧前沉积主要由三花岩群包括王家坝组砂岩以及由泥岩、砂岩和中酸性火山岩变质而成的片岩、片麻岩和石英岩组成.安康弧前盆地具有明显的深海扇沉积特征梅子垭群和大贵坪组;⑧南秦岭弧后盆地系,包括后龙门山的茂县群和上古生界及三叠系,大巴山的洞河群和部分耀岭河群的火山岩;⑨南秦岭弧后陆坡带,只保留大巴山弧后陆缘,是高川-毛坝以南的下古生界;⑩南秦岭前陆褶冲带,包括龙门山北段、米仓山和大巴山前陆褶冲带.三带形成于印支-燕山期,但构造线不同,且在出现的时间上,由西到东由早到晚;(11)三叠纪残余海盆;(12)中-新生代走滑拉分和断陷盆地;(13)基底断块.     

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

The Upper Jurassic Tordillo Formation is exposed along the western edge of the Neuquén Basin (west central Argentina) and consists of fluvial strata deposited under arid/semiarid conditions. The pebble composition of conglomerates, mineralogical composition of sandstones and pelitic rocks, and major- and trace-element geochemistry of sandstones, mudstones, and primary pyroclastic deposits are evaluated to determine the provenance and tectonic setting of the sedimentary basin. Conglomerates and sandstones derived almost exclusively from volcanic sources. The stratigraphic sections to the south show a clast population of conglomerates dominated by silicic volcanic fragments and a predominance of feldspathic litharenites. This framework composition records erosion of Triassic–Jurassic synrift volcaniclastic rocks and basement rocks from the Huincul arch, which was exhumed as a result of Late Jurassic inversion. In the northwestern part of the study area, conglomerates show a large proportion of mafic and acidic volcanic rock fragments, and sandstones are characterised by a high content of mafic volcanic rock fragments and plagioclase. These data suggest that the source of the sandstones and conglomerates was primarily the Andean magmatic arc, located west of the Neuquén Basin. The clay mineral assemblage is interpreted as the result of a complex set of factors, including source rock, climate, transport, and diagenesis. Postdepositional processes produced significant variations in the original compositions, especially the fine-grained deposits. The Tordillo sediments are characterised by moderate SiO₂ contents, variable abundances of K₂O and Na₂O, and a relatively high proportion of ferromagnesian elements. The degree of chemical weathering in the source area, expressed as the chemical index of alteration, is low to moderate. The major element geochemistry and Th/Sc, K/Rb, Co/Th, La/Sc, and Cr/Th values point to a significant input of detrital volcanic material of calcalkaline felsic and intermediate composition. However, major element geochemistry is not useful for interpreting the tectonic setting. Discrimination plots based on immobile trace elements, such as Ti, Zr, La, Sc, and Th, show that most data lie in the active continental margin field. Geochemical information is not sufficiently sensitive to differentiate the two different source areas recognized by petrographic and modal analyses of conglomerates and sandstones.     

Geochemistry of Proterozoic volcanic and granitic rocks from the Gold Hill-Wheeler Peak area,northern New Mexico

Early Proterozoic supracrustal and plutonic rocks from the Gold Hill-Wheeler Peak area in northern New Mexico define three populations: amphibolite—diorite—tonalite, hornblendite—cumulus amphibolite and felsic volcanics and porphyries. Also present are mid-Proterozoic granites. Amphibolites are similar in Ti, Zr, Cr, Ni and REE contents to young calc-alkaline and arc basalts and diorites and tonalites are similar in composition to young andesites and to high-Al₂O₃ tonalites, respectively. Felsic volcanics resemble young felsic volcanics from mature arc systems in their immobile-element contents. Geochemical model studies suggest that the amphibolites, hornblendites, diorites and tonalites are related by progressive fractional crystallization of a hydrous parent tholeiite magma produced from partial melting of undepleted lherzolite. Amphibolites represent parent tholeiites modified by olivine removal. Hornblendite is an early solid residue comprised chiefly of hornblende, clinopyroxene, and olivine; diorite and cumulus amphibolite represent respectively residual solid (clinopyroxene, plagioclase, hornblende) and liquid, after 50% crystallization. Tonalite represents a residual liquid after 80% crystallization. Felsic volcanic rocks are produced by partial melting of a tonalite or diorite source with granulite-facies mineralogy in the lower crust. Granites have a similar origin to felsic volcanics although requiring an inhomogeneous source with the presence of residual hornblende or garnet.The calc-alkaline igneous rocks in the Gold Hill-Wheeler Peak area suggest the presence of an arc system in northern New Mexico during the Early Proterozoic. The fact that these rocks interfinger with and are overlain by mature clastic sediments favors a model in which a continental arc system is uplited, eroded and buried by cratonic sediments from the north.     

Petrographic and geochemical characteristics of Paleogene turbidite deposits in the southern Aegean (Karpathos Island,SE Greece): Implications for provenance and tectonic setting

The provenance and depositional setting of Paleogene turbidite sediments from the southern Aegean are investigated using petrography and whole-rock geochemistry. Petrography indicates that Karpathos Island turbidites are consisting of compositionally immature sandstones (graywackes–litharenites) derived from igneous (plutonic–volcanic), sedimentary, low-grade metamorphic and ophiolitic sources. The studied sediments probably reflect a mixing from an eroded magmatic arc and from quartzose, recycled sources. Major and trace element data are compatible with an acidic to mixed felsic/basic source along with input of ultramafic detritus and recycling of older sedimentary components. Geochemical data also reveal that the sediments have undergone a minor degree of weathering and no significant sediment recycling. Chondrite-normalized REE plots show a light REE enrichment (LaN/YbN ca. 7) and absence of significant negative Eu anomalies, indicating provenance from young undifferentiated arc material with contribution from an old upper continental crust source. Turbidite sedimentation probably took place in a continental island arc depositional setting as a result of subduction of a branch of Neotethys beneath a continental fragment of the Anatolide domain in Early Tertiary times. The relation of Karpathos turbidites with the Pindos foreland basin (Gavrovo and Ionian Zones of western Greece) remains problematic.     

Sedimentary record of explosive silicic volcanism in a Cretaceous deep-marine conglomerate succession, northern Antarctic Peninsula

Lower Cretaceous conglomeratic strata exposed on southern Sobral Peninsula were deposited on a deep‐marine apron in the back‐arc Larsen Basin close to its faulted boundary with the Antarctic Peninsula magmatic arc. The succession is dominated by amalgamated beds of clast‐supported conglomerate, which, together with minor intercalated sandstones, consist of varied, but largely basaltic to andesitic, volcanic material and clasts derived from the Palaeozoic–Triassic (meta)sedimentary basement of the arc. Most of the volcanic clasts are thought to have been derived from lithified volcanic successions or older synvolcanic deposits, rather than from sites of coeval eruption. These mixed‐provenance strata enclose a number of intervals, consisting mainly of inverse–normally graded conglomerate and graded–stratified pebbly sandstone, in which the sand fraction is dominated by crystals and vitric grains considered to have been redeposited in the immediate aftermath of explosive silicic arc volcanism. Like syneruption deposits on non‐marine volcaniclastic aprons, these intervals are more sand‐prone than the enclosing strata and appear to show evidence of unusually rapid aggradation. Plagioclase from one such interval has yielded ⁴⁰Ar/³⁹Ar ages concordant at ≈121 Ma, similar to those obtained from the non‐marine Cerro Negro Formation, deposited within the magmatic arc. It is suggested that the two successions can be viewed as counterparts, both recording a history of mainly basaltic to andesitic volcanism, punctuated by relatively infrequent, explosive silicic eruptions. Whereas the Cerro Negro Formation consists mainly of syneruption deposits, most of the volcaniclastic material delivered to the eruption‐distal, deep‐marine apron appears to have been derived by normal degradation processes. Only rare silicic eruptions were capable of supplying pyroclastic material rapidly enough and in sufficient quantities to produce compositionally distinct syneruption intervals.     

Origin of the Torlesse Terrane and Coeval Rocks,North Island,New Zealand

On the basis of differing areal extent, age, petrographic modes, and bulk chemical composition, the sandstones of the northern quarter of the Torlesse terrane are subdivided into four new petrofacies. A comparison of these petrofacies with existing South Island Torlesse classifications indicates continuation of the Triassic Rakaia subterrane and Late Jurassic–to–early Cretaceous Pahau subterrane into the central part of the North Island (as Axial-A and Axial-B petrofacies, respectively). The Waioeka petrofacies defines a new and provisional Late Jurassic-to–early Cretaceous Waioeka subterrane that is not present in the South Island. The Omaio petrofacies is common to deformed Albian basement sequences in the Torlesse of both islands, and in the Houhora Complex of Northland.

The composite Torlesse terrane evolved by Early Jurassic accretion of allochthonous Rakaia rocks followed by parautochthonous deposition of Pahau and Waioeka sandstones. Waioeka sandstones are compositionally similar to sandstones in the coeval eastern Waipapa terrane, but may have been dextrally displaced from their original depositional site by up to 300 km since the middle Cretaceous.