The effects of source lithology, transport, deposition and sampling scale on the composition of southern California sand

The Transverse Ranges of southern California represent an uplifted and variably dissected Mesozoic magmatic arc, and Mesozoic to Holocene sedimentary and volcanic strata deposited in convergent and transform tectonic settings. Modern sand within part of the Western Transverse Ranges represents: first-order sampling scale of the Santa Monica and the San Gabriel Mountains; second-order sampling scale of the Santa Clara River draining both mountain ranges; and third-order sampling scale of the beach system between the mouth of the Santa Clara River and the eastern Santa Monica Mountains, and turbidite sand of the Hueneme-Mugu submarine fan. Source lithology includes plutonic and metamorphic rocks of the San Gabriel Mountains, and sedimentary and volcanic rocks of the Santa Monica Mountains. First-order sands have large compositional variability. Sand from local coastal drainage of the Santa Monica Mountains ranges from basaltic feldspatholithic to quartzofeldspathic. Sand of the San Gabriel Mountains local drainages has three distinct petrofacies, ranging from metamorphiclastic feldspatholithic to mixed metamorphi/plutoniclastic and plutoniclastic quartzofeldspathic. Second-order sand is represented by the main channel of the Santa Clara River; the sand has an abrupt downstream compositional change, from feldspathic to quartzofeldspathic. Third-order sand (beaches and deep-sea turbidite samples) of the Santa Monica Basin is quartzofeldspathic. Beach sand is more quartz-rich than is Santa Clara river sand, whereas turbidite sand is more feldspar-rich than is beach sand. Deep-sea sand has intermediate composition with respect to second-order samples of the Santa Clara River and third-order samples of the beach system, suggesting that (1) the Santa Clara River is the main source of sediments to the marine environment; and (2) local entry points from canyons located near local drainages may generate turbidity currents during exceptional flood conditions. Petrologic data of modern sand of the study area are highly variable at first- and second-order scale, whereas third-order sand is homogenized. The homogenized composition of deep-marine sand is similar to the composition of most ancient sandstone derived primarily from the Mesozoic dissected magmatic arc of southern California. This study of the Western Transverse Ranges illustrates the effects of source lithology, transport, depositional environment, and sampling scale on sand composition of a complex system, which provides insights regarding actualistic petrofacies models.     

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).     

It all comes out in the wash: Actualistic petrofacies modeling of temper provenance,Tonto Basin,Arizona, USA

Prehistoric potters frequently tempered their pottery with sand, the provenance of which can be established petrographically. In the Tonto Basin of central Arizona, the bedrock geology is highly variable, giving rise to geographically unique sands. Zones of sands with similar compositions can be modeled at an archaeologically relevant scale. Here we use the actualistic petrofacies concept, the Gazzi–Dickinson point‐counting technique, and multivariate statistics to create a petrofacies model, then apply it to sand‐tempered utilitarian pottery recovered from three Tonto Basin project areas. Data analysis reveals strong temporal and spatial ceramic production and consumption patterns. Production of pottery for exchange was established in at least one petrofacies ca. A.D. 600–950, with increasing specialization through time. By ca. A.D. 1150, corrugated wares had been added, and specialized production by ware was established in two petrofacies. Provenance evidence suggests different regional affiliations for groups in different parts of the basin. © 2001 John Wiley & Sons, Inc.     

Source-area controls on the composition of beach and fluvial sands on the southern side of the Gibraltar Strait and Western Alboran Sea (Flysch Basin, Internal and External, Domains, Northern Rif Chain)

The southern side of Gibraltar and the Western Alboran Sea of the northern Rif coasts and rivers provide a natural field laboratory for sampling modern sand at different scales: small catchment basins (first order) and rivers draining mountain belts (second order). The Rifian chain represents a deformed and uplifted thrust-belt and related forelands composed of Palaeozoic nappes, metamorphic and plutonic basement, and their sedimentary Mesozoic and Cenozoic siliciclastic and carbonate cover, respectively. The present physiography of the Rif Chain is shaped by a rugged mountainous relief drained by different scale catchment basins that supply the nearby coastal and marine deep-sea environments. The analysis of the composition of modern fluvial and beach sands is useful for the interpretation of transported sediments by surface processes from the continent toward coasts and later to deep-water environments.Modern beach and fluvial sands of the southern side of Gibraltar and the Western Alboran Sea display three distinct petrologic littoral provinces, from the east to the west and from the north to the south, respectively, designated as: (i) the Tangier–Bel Younech Littoral Province with 90% of sand derived from erosion of Flysch Nappes (Flysch Basin Domain); (ii) the Bel Younech–Sebta Littoral Province with 64% of sand fed mainly by the metamorphic Units of Upper Sebtides and (iii) the Sebta–Ras Mazari Littoral Province with 74% of sand supplied from the epimetamorphic Palaeozoic Ghomaride Nappes and Alpine cover rather than Mesozoic and Cenozoic sedimentary successions of the “Dorsale Calcaire” Units. Comparison of detrital modes of fluvial and coastal marine environments highlights their dispersal pathways and drainage patterns of actualistic sand petrofacies.     

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.     

漠河盆地上侏罗统物源分析及其地质意义

为了探讨晚侏罗世漠河盆地的构造类型,笔者等对其物源特征进行了系统分析。通过古水流分析、母岩成分分析和源区构造背景分析认为:①晚侏罗世漠河盆地的物源来自南北两个方向;②北部物源区位于西伯利亚板块南缘,为蒙古—鄂霍茨克造山带,母岩成分主要为花岗岩、变质岩、中酸性火山岩、中基性火山岩和沉积岩;③南部物源区位于大兴安岭北部,为下伏板块基底,母岩成分主要为花岗岩、变质岩和沉积岩;④北部造山带物源区的构造背景为早中生代的活动大陆边缘。晚侏罗世漠河盆地具有典型前陆盆地的双向物源特征,一方面来自北部造山带,一方面来自盆地下伏板块基底。根据物源特征、区域大地构造背景和俄罗斯上阿穆尔盆地(黑龙江在俄罗斯称为阿穆尔河)有关资料认为晚侏罗世漠河盆地可能为漠河—上阿穆尔周缘前陆盆地的南半部分,其形成和演化受蒙古—鄂霍茨克造山带制约。     

岩石大地构造学说的兴起、没落与新生

在板块构造环境中形成的岩石组合(rock assemblages)被称为岩石大地构造组合(petrotectonic assemblages)。岩石大地构造学说创立于20世纪70~80年代,经历了兴起、没落与新生的曲折过程。研究表明,由Pearce等学者创立的岩石大地构造学说的理论基本正确,概括来说,洋中脊、洋岛和岛弧三大构造背景的源区是不同的,这是玄武岩判别图的理论基础。目前的情况是,三大构造背景源区不均一性可能比早先认为的更复杂。由于不同构造环境源区的复杂性,早先的玄武岩判别图采用的是抽样数据、典型地区、精确数据,显然不适合全球数据海量积累的情况,遂使玄武岩判别理论遇到了瓶颈。我们的研究发现,不同构造背景的所有岩石、矿物(包括玄武岩、苦橄岩、辉长岩、堆晶岩、橄榄石、单斜辉石、尖晶石等)几乎都保留了不同构造背景的“基因”信息,并且大多可采用大数据方法予以识别,遂使岩石大地构造学说焕发了青春。虽然本文提出了不同构造环境存在不同“基因”的假说,按照大数据方法判别的效果显著增加了。但是,这个不同构造背景的“基因”究竟是什么仍然不清楚。此外,上述“基因”假说的理论解释还非常不足,需要今后进一步探讨。     

Rare earth elements of the Altar Desert dune and coastal sands, Northwestern Mexico

Twenty-one surficial sand samples from the Altar Desert coastal and desert dune systems were analysed for rare earth elements (REE) content. This was done to observe the provenance signatures for four strategic dune localities near the Colorado River Delta, the El Pinacate dune fields, and the beaches of the north of the Gulf of California in the state of Sonora, Mexico. Our goals are to show which mechanisms (i.e., aeolian, marine) exert more influence on the composition of the Altar Desert dune sands. This study also shows the usefulness of REE spatial distribution to determine the relative mobility of the sand. Some sand samples from the dune systems in San Luis Río Colorado (SLRC), Golfo de Santa Clara (GSC), and Puerto Peñasco (PP) displayed dissimilar REE concentrations with respect to the rest of the sand samples from the same sites. These differences can be related to short aeolian transport distance in the sands with high REE concentrations and long aeolian transport distance in the sands with low REE concentrations. Besides, high REE concentration in the sands might be due to their closeness to the Colorado River Delta sediments and to recycled sands derived from granitic rocks. In contrast, all the sand samples from the El Pinacate (EP) site have similar REE concentration values, suggesting that the El Pinacate dune sands are influenced by more selective aeolian processes and less diverse heavy mineral content. The Altar Desert dune sands are derived from granitic sources eroded by the Colorado River. Our results also indicate that the Altar Desert dune sands are low in heavy mineral content (with the exception of Fe and Ti bearing minerals) and enriched in carbonates with phosphates (especially at the PP site) yielding poor correlations between REE and major element concentrations. The REE geographical distribution values in the Altar Desert dune sands indicate that light and heavy REE concentration values are related to aeolian transport, maturity of the sands, their low weathering rates, proximity of the source rocks, and the biogenic debris input from beach sands into the dune.     

A review of wave-dominated clastic shoreline deposits

Barrier islands, beaches, wave-dominated deltas and chenier plains probably form more than one-fifth of present coasts, and constitute an even larger proportion of the subsiding lowland coasts likely to be preserved in the stratigraphic record. The geometry, vertical sequences and composition of their deposits are the result of: (1) shoreline behaviour, whether transgressive, regressive or stable; (2) physical processes, particularly the relative intensities of waves and tides; (3) climate; (4) global tectonic setting; and (5) topography of underlying deposits.Published interpretations indicate that equivalent deposits are abundant in the geological record. Sandstones are mostly quartzose, cemented by carbonates and quartz, and have sheet or linear geometries. Ancient deposits can provide evidence of palaeogeography and some are economically significant, particularly as hydrocarbon reservoir rocks.Four end-member sedimentation models are proposed for the interpretation of ancient deposits: (1) transgressive sheet sands associated with deltas; (2) non-deltaic transgressive sheet sands; (3) regressive sheet sands; and (4) linear sand bodies. Probable depositional settings, geometries, characteristics of deposits and palaeocurrent patterns are discussed.     

Provenance of the Pythian Cave conglomerate,northern California: implications for mid-Cretaceous paleogeography of the U.S. Cordillera

Provenance analysis of middle Cretaceous sedimentary rocks can help distinguish between disparate tectonic models of Cretaceous Cordilleran paleogeography by establishing links between sediment and source, as well as between currently separated basins. This study combines new detrital zircon age data and compositional data with existing provenance data for the Pythian Cave conglomerate, an informally-named unit deposited unconformably on the eastern Klamath Mountains, to test possible correlations between the Pythian Cave conglomerate and similar-age deposits in the Hornbrook Formation and the Great Valley Group. These provenance results indicate that restoring Late Cretaceous clockwise rotation of the Blue Mountains adds a significant sediment source for Cretaceous basins previously associated with only the Klamath Mountains (e.g., the Pythian Cave conglomerate and Hornbrook Formation) or a combined Klamath-Sierran source (e.g., Great Valley Group). Comparison of the Pythian Cave conglomerate with the Klamath River Conglomerate and the Lodoga petrofacies suggests that the Pythian Cave conglomerate system was separate from the nearby Hornbrook Formation and was probably related to the Lodoga petrofacies of the Great Valley Group.     

Provenance of coastal dune sands along Red Sea,Egypt

Texture, mineralogy, and major and trace element geochemistry of 26 coastal dune sand samples were studied to determine the provenance and tectonic environment of two dune fields close to the beaches of Safaga (SF) and Quseir (QS) at the Egyptian Red Sea coast. Onshore winds generate fine, moderate, moderately-well to well-sorted, coarse-skewed to near-symmetrical dune sands with mesokurtic distributions. Winds pick up and transport grains from nearby beach sands and alluvial deposits into a wide Red Sea coastal plain at the border of the beach. The mineralogical (Qt–Ft–Lt) and geochemical composition of the sands, indicate that SF and QS coastal dune sands are mature and influenced by quartz-rich sands. The average CIA values in SF and QS coastal dune sands are low relative to the range of the PAAS, suggesting an arid climate and a low intensity of chemical weathering. The SF and QS coastal dune sand samples are plotted in the recycled orogen and partly in craton interior fields suggesting recycled older sedimentary and partly metamorphic-plutonic sources. The high content of quartz with shell debris and carbonates in coastal dune sands support the recycled sedimentary beach and alluvial sand sources. The dominance of heavy minerals like amphiboles (hornblende) and biotite in the coastal dune sands also supports the effect of metamorphic-plutonic source rocks. The new tectonic discriminant-function diagrams suggest that the coastal dune sands were deposited in a passive margin of a synrift basin. The results provide a good evidence for the extension in the Red Sea rift system during Oligocene-post Pliocene, which is consistent with the general geology of Egypt.     

胶东半岛构造-岩相形式及玲珑-焦家式金矿的构造动力成岩成矿地质特征研究

玲珑-焦家式金矿是指胶东前寒武变质岩系内中生代花岗岩含金剪切带石英脉-绢英质蚀变岩金矿类型。由于赋矿及成矿构造性质不同,该类矿床可分为明显有别的两种典型矿床类别。区域成矿作用受中生代“N”形断陷构造-岩相形式与基底反“S”弧形断褶-变质岩相形式的复合控制。成矿是沉积→变质→交代和重熔作用发生发展的综合结果。开展矿源岩系(序)列研究,划分成矿花岗岩类为剪切、挤压及引张构造-岩浆岩相,并提出了室内外鉴别标志。 作者在对各层次岩石结构、形变与成分、相变等相关的地质事实研究基础上进一步探讨了成因,侧重以“构造作用力影响成岩成矿静水压力”的观点建立了理论、数学模型,实测了古应力、古压力及其它成矿物理化学参数,从而论证了构造附加静水压力是变形岩石发生相应相变的主要动力因素,构造附加静水压力不仅是使变形岩石体变的物理量,而且是影响、制约其中成岩成矿化学过程的物理化学参变量。     

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.     

Great Victoria Desert: Development and sand provenance

Sands of the Great Victoria Desert, south‐central Australia, can be divided into three main groups on the basis of their physical and chemical characteristics (colour, grainsize parameters, mineralogy of heavy‐mineral suites, quartz oxygen isotopic composition, zircon U–Pb ages). The groups occupy the western, central and eastern Great Victoria Desert respectively, boundaries between them corresponding approximately to changes in the underlying rocks associated with the Yilgarn Craton to Officer Basin to Arckaringa Basin. Several lines of evidence suggest derivation of the sands mainly from local bedrock with very little subsequent aeolian transport. Ultimate protosources for the sands, each in order of importance, are: western Great Victoria Desert—Yilgarn Craton, Albany‐Fraser Orogen, Musgrave Complex; central Great Victoria Desert—Musgrave Complex; eastern Great Victoria Desert—Gawler and Curnamona Blocks, Adelaide Geosyncline, Musgrave Complex. Sediment from the Adelaide Geosyncline includes in addition an ‘exotic’ component from Palaeozoic sedimentary rocks probably derived mainly from Antarctica. Sediment transport of several hundred kilometres from these protosources to the sedimentary basins was dominantly by fluvial, not aeolian, means. Post‐Tertiary aeolian transport or reworking has been minimal, serving only to shape sand eroded from underlying sedimentary rocks or residual products of local basement weathering into the current dunes.     

Late Cretaceous redbeds from the Gan-Hang Belt in Southeast China: petrography and geochemistry implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in Southeast China were generated by crustal extension associated with subduction of the Palaeo-Pacific plate during the late Mesozoic. Compared with adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstones from the Late Cretaceous Guifeng Group in the Yongchong Basin, Southeast China. Detrital grains are subangular to subrounded, poorly sorted, and rich in lithic fragments. Variable Chemical Index of Alternation values (59.55–79.82, avg. 66.79) and high Index of Compositional Variability (ICV) values (0.67–3.08, avg. 1.40) indicate an overall low degree of chemical weathering and rapid physical erosion of source rocks. Such features are consistent with an active extension tectonic setting. Other chemical indices (e.g. Al₂O₃/TiO₂, Th/U, Cr/Th, Th/Sc, Zr/Sc) also suggest significant first-cycle sediment input to the basin and a dominant felsic source nature. Thus, the Guifeng Group possibly underwent moderate to low degrees of weathering upwards. Sandstone framework models and geochemical characteristics suggest the provenance was likely a combination of passive margin (PM) and active continental margin (ACM) with minor continental island arc (CIA) tectonic settings. Sediment derivation from Neoproterozoic metamorphic rocks and Cambrian to Triassic granitoids indicates PM provenance, whereas sediments derived from Jurassic to Cretaceous granitoids suggest ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben basin under the back-arc extension regime when Southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.     

Some tectonic implications of sandstone petrofacies in the Coffs Harbour Association,New England Orogen,New South Wales

The Coffs Harbour Association, New England Orogen, consists of thick, monotonous units of Late Palaeozoic greywacke, laminated siltstone and mudstone, and massive argillite. The rocks of the association have a common provenance, being derived predominantly from a volcanic arc source consisting of mainly dacite, with minor andesite and rhyolite. The Coramba beds in the Coffs Harbour Block are divided into four petrofacies based on QFL data and the occurrence of detrital hornblende. Upwards, the petrofacies are: A—volcanolithic, B—feldspathic, C—horn‐blende‐feldspathic, D—hornblende‐volcanolithic. The petrofacies and vertical variation in non‐volcanic detritus indicate minor erosion and exposure of a non‐volcanic source, followed first by recommencement of volcanism, penecontemporaneously with sedimentation, then further erosion of the non‐volcanic source area. There was little temporal change in the character of volcanic detritus shed from the source area. Equivalents of the four petrofacies are recognised in other blocks of the association, although because of structural complexity, a complete A‐D sequence has not been found. The Coffs Harbour sandstones are similar to sands in modern ocean basins derived from an arc system of either continental margin or island arc type. The sandstones are not similar to recycled orogenic provenances, such as found in accretionary prisms or trench‐slope basins; the compositions suggest that the sandstones were deposited in either a forearc or backarc setting.     

大兴安岭索伦地区哲斯组碎屑岩地球化学特征和锆石U-Pb年龄对沉积物源属性约束

本文以内蒙古索伦地区好仁镇出露良好的中二叠世哲斯组碎屑岩为研究对象,利用碎屑岩地球化学特征和锆石年代学等示踪技术分析哲斯组沉积物源的年代和性质。测年结果显示,哲斯组砂岩碎屑锆石U-Pb年龄分为6组：282~244、352~294、468~379、513~482、802~580、2 500~1 800 Ma。碎屑岩地球化学具有高SiO₂和低Fe₂O₃、FeO、MgO质量分数的特点,说明其母岩为长英质岩石。稀土、微量元素特征及其构造图解显示,哲斯组沉积物源区多具有大陆岛弧型和活动大陆边缘型的特点,少量反映被动大陆边缘的特点。对比研究显示,沉积物源区所反映的大陆岛弧型和活动大陆边缘型的构造环境是不同时期佳-蒙地块拼合碰撞和古亚洲洋南支北向俯冲的结果,少量所反映的被动大陆边缘环境可能是部分基底岩石形成时构造环境的响应。鉴于研究区位于兴安地块东南部,远离华北地块,且哲斯组碎屑岩多为长石岩屑砂岩、岩屑长石砂岩、岩屑砂岩,其结构成熟度和成分成熟度较低,显示近源沉积的特点,与其同期的火成岩均显示岛弧或活动大陆边缘的构造背景,据此推测哲斯组应形成于古亚洲洋南支向佳-蒙地块俯冲形成的弧后或弧间盆地环境,物源主要来自于佳-蒙地块南缘与岛弧或活动大陆边缘有关的火成岩,少量来自于佳-蒙地块的古老基底。     

K_2O/(Na_2O+CaO)--SiO_2/Al_2O_3:一个用于推断复理石形成时板块构造背景的判别图

沉积岩中的主要氧化物:SiO_2,Al_2O_3,K_2O,Na_2O,CaO的丰度及其SiO_2/Al_2O_3,K_2O/(Na_2O+CaO)值的变化与物源区类型和板块构造背景密切相关。因此,利用砂岩的化学成分可以判断其形成时的板块构造背景。笔者重点研究了造山带中重要且分布广泛的复理石中杂砂岩的化学成分及其与板块构造背景之间的关系,绘制了用于推断复理石形成时的板块构造背景的SiO_2/Al_2O_3——K_2O/(Na_2O+CaO)双变量判别图。经对15组不同时代复理石的180个化学成分进行投影分析,能有效地分开不同板块构造背景下的复理石。     

新疆西博格达山周缘地区中新生代沉积岩源区和构造背景

沉积岩中的碎屑组分及相对稳定的微量元素和稀土元素可以反映物源信息,广泛应用于沉积源区的确定和构造背景的分析。本文通过对西博格达山周缘地区侏罗纪至古近纪沉积地层的碎屑岩岩石学和微量元素与稀土元素分析,揭示盆地沉积岩的源岩来自上地壳,岩性以长英质岩石为主,混合部分安山质及基性岩石。源区构造背景应为岩浆弧和再循环造山带,其中岩浆弧起主导作用。本文的分析结果为研究博格达山的构造演化过程及其机制提供了有力的证据,并且对准噶尔盆地油气勘探具有一定指导意义。     

哀牢山缝合带中两类火山岩地球化学特征及其构造意义

在哀牢山构造带的哀牢山断裂与花山－雅邑断裂之间出路大量不同构造环境的岩浆岩。除双沟蛇绿岩外,尚发现有景东火山岩和墨江火山岩。地球化学研究表明,景东火山岩具有类似于富集型洋中脊玄武岩（Ｐ－ＭＯＲＢ）地球化学特征,形成于以双沟蛇绿岩为代表的哀牢山洋盆先期的陆内裂谷构造环境;墨江火山岩具有岛弧火山岩地球化学特征,形成于哀牢山洋盆向西俯冲消减作用下的岛弧构造环境。景东裂谷型火山岩和墨江岛弧火山岩分别代表了