Petrology of glacial sand: implications for the origin and mechanical durability of lithic fragments

The petrology of first cycle (unmodified) and second cycle (reworked) sand at the termini of eleven valley glaciers eroding coarse- to fine-grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands. First cycle sands are coarse- to medium-grained, poorly sorted, fine-skewed, non-modal lithic sands with an average composition of Q₂₁F₆L₇₃. Grain-size distributions do not vary with composition or source rock types, although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources. By contrast, second cycle sands are medium-grained, poorly sorted, fine- to coarse-skewed arkosic to lithic sands with an average composition of Q₁₉F₄₀L₄₁, and contain fewer lithic fragments than do first cycle sands. We propose that the origin, mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness, and the particular stress field of the transporting medium. As a result of abrasion, glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain-size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock. Upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point-loading during saltation in addition to shear stress. Point-loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness, so the lithic fragments break into smaller monomineralic grains and/or lithic fragments. Lithic fragments of very coarse sand-size are abundant in first cycle sands, thus refuting, at least for glacial sands, the widely held view that grains of this size are deficient in nature. The presence of fewer grains of this size in second cycle sands indicates that very coarse sand-size lithic fragments can survive recycling, but in greatly reduced proportions due to breakage.     

Mineralogy of arc-derived sediment: siliciclastic sediment on the insular shelf of Puerto Rico

Siliciclastic sediments on the insular shelf of Puerto Rico are derived from a volcanic arc. The sands are feldspatholithic and subquartzose. They confirm predictions about sandstones of arc derivation in the following ways. The quartz content is low, usually less than 25 %. Feldspar is more abundant than quartz, but less so than lithic fragments. If mud rip-up clasts are excluded from the lithic suite, the ratio V/L (volcanic lithic grains to total unstable lithic grains) assumes values near 0.75. Most of the volcanic lithic grains have textures suggesting intermediate volcanics as source rocks. Ratios of plagioclase to total feldspar are high. Values of the parameter C/Q (stable lithic fragments to total lithic grains) are higher than expected for sandstones of volcanic provenance. Four compositionally and areally distinct assemblages of clay minerals are recognized. Clay suites characterized by major concentrations of both halloysite and smectite are found off the north-west coast. Sediments off the north-central and north-east coasts contain major concentrations of halloysite and lesser amounts of smectite, illite and chlorite. Major concentrations of kaolinite are present off the south-east coast. An assemblage of poorly crystallized smectite with lesser amounts of illite and chlorite occurs off the south-west and south-central coasts. Variation in the mineralogic composition of clays around the perimeter of the island reflects changes in the intensity of chemical weathering and the type of rock exposed in the immediate source area and, in instances, either early diagenetic alteration or, more likely, sorting within the clay fraction by currents on the shelf.     

NON-GREYWACKE "TURBIDITE" SANDSTONES IN THE WELSH GEOSYNCLINE

In contrast with the commonly accepted notion regarding ancient turbidites, non-greywacke sandstones are not uncommon in the typically graded turbidite facies of the Cambrian and Silurian sediments in north Wales. The sandstones are the arkosic and lithic types of PETTIJOHN (1957) or the feldspathic and lithic arenites of GILBERT (1954) and occur at the bottom of graded beds when the grain size tends to be above medium grade. Petrological features suggest that debris forming the sandstones in north Wales was not significantly modified during transportation and original provenance characters are well preserved. The occurrence of such sandstones implies that: (1) the current concept of ancient graded greywackes in the turbidite facies should be revised; (2) non-greywacke sandstones in ancient turbidites are comparable in petrological features to recent deep-sea sands; (3) these sandstones are important in connection with the origin of the clay matrix in greywacke.     

Chromian spinel-rich black sands from eastern shoreline of Andaman Island,India: Implication for source characteristics

Black sands rich in chromian spinel commonly occur in pockets along the eastern shoreline of Andaman Island where various types of peridotites and volcanics belonging to the Andaman ophiolite suite are exposed in close vicinity. The chemistry of these detrital chromian spinels has been extensively used here in identifying the source rocks vis-à-vis deciphering the source characteristics. The composition of the chromian spinels (Cr#: 0.20–0.88, Mg#: 0.26–0.77, Al ₂ O ₃: 5.04–48.21 wt.%, TiO ₂: up to 1.39 wt.% and Fe ²⁺/Fe ³⁺: 1.73–9.12) varies widely signifying multiple sources, of which mantle peridotites and volcanic rocks are relevant in an ophiolitic terrain. The volcanic chromian spinels are relatively fresh, commonly euhedral, sometimes with compositional variations, and contain inclusions in contrast to the mantle peridotitic chromian spinels which are rounded, extensively fractured, and altered. We used a number of geochemical bivariate plots in order to know the provenance protoliths. The volcanic chromian spinels show geochemical characters of MORB, related to spreading centers (either MOR or back-arc) and also boninites/arc-tholeiites, related to active subduction. On the other hand, the peridotitic spinels indicate partially depleted lherzolite and depleted harzburgite source of the ophiolite suite.     

Petrography and provenance of subsurface Neogene sandstones of Bengal Basin, Bangladesh

This study deals with petrography and provenance of the Neogene reservoir sandstones encountered in the Kailas Tila, Titas, Bakhrabad and Shahbazpur Gas Fields of Bengal Basin. Framework grains are sand-sized to silt-sized particles of mainly detrital origin. The most common detrital grains are quartz, feldspars, and rock fragments. Mica occurred as minor and non-opaque heavy minerals found as minor accessories. Among the main detrital framework grains, quartz constitutes 51–60%, feldspar 3–15%, lithic fragments 8–22%. Sandstones encountered in the studied wells have been classified as sublithic arenite, feldspathic arenite and lithic arenite in order of abundance. Different triangular plots reveal that the Neogene sandstones of the studied wells exhibit a quartzolithic composition, low feldspar, very low volcanic grains and abundant sedimentary and low grade metamorphic lithic clasts indicating that the sands were derived from quartzose recycled orogen province, such as a fold thrust province or a collision suture zone. This study suggests that either the eastern Himalayas or Indo-Burman Ranges might act as the source of the sandstones of the studied wells of the Bengal Basin.     

应用电子探针研究蒙其古尔铀矿床含矿砂岩岩石学特征及铀矿物分布规律

应用精准的砂岩碎屑成分定量分析,可以为源区及沉积盆地分析等相关性质提供重要依据。本文在野外观察描述砂岩宏观特征的前提下,利用显微镜结合电子探针分析手段,对新疆蒙其古尔铀矿床中的含矿砂岩进行研究,探讨含矿砂岩岩石学特征及其中铀矿物的分布规律。结果表明：研究区含矿砂岩成分成熟度指数介于0.56~1.08,平均指数为0.78,即成分成熟度、结构成熟度低,成岩作用强烈;物源为一套混合型再旋回造山带区向过渡再旋回造山带区,最后向岩屑再旋回造山带区过渡的演变过程。碎屑物源主要来源于盆地南缘蚀源区石炭-二叠系中酸性火山岩,以及盆地周边那拉提山北缘、科古琴山、博努科努山南缘分布的一系列晚泥盆-早石炭世大哈拉军山组火山岩和少量古生代花岗岩。铀矿物类型主要为沥青铀矿、铀石及钛铀矿,其次发现有少量吸附态铀;背散射图像显示铀矿物的分布与炭屑、黄铁矿、金红石等密切共生。     

Major Characteristics of the Lajishan Orogenic Belt of the South Qilian Mountains and Its Geotectonic Attribute

The Lajishan orogenic belt is one of the E-W-trending Caledonian erogenic belts within the Qinling-Qilian orogenic system. It was formed upon the Jiningian basement by intensive taphrogenesis. Its major characteristics comprise the prominent faulting along the north and south boundaries, the highly complicated petrological and petro-geochemical features of the volcanic rock series, and the development of a new type of ophiolite suite. In terms of tectonic analysis and the sequential analysis of tectonic settings of magmatic rocks, it is suggested that the Lajishan orogenic belt has undergone a complete "opening-closing" cycle, which can be further divided into 3 second-order "opening-closing" cycles. The composite characteristics of the "opening-closing" movement show that Laji Mountain is a typical fault orogenic belt. The fault orogenic belt is one of the most important types of intracontinental orogens. It is of critical theoretical and practical significance to summarize the characteristics and the     

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.     

川东北元坝地区须家河组石英砂岩沉积与储层特征

在川东北元坝及周边地区首次发现纯净的石英砂岩沉积,石英砂岩厚度10~15 m左右。研究表明,石英砂岩主要以夹层形式出现在须一、二段,但沉积环境相当特殊,主要为海泛或湖泛早期沉积的滩坝砂体,不属于须家河组广泛发育的三角洲沉积体系,石英砂体呈近东西向展布,明显受到米仓山-大巴山前陆前渊带的控制。石英砂岩以粉-细砂为主,分选好,磨圆度高,杂基含量低,具有较高的成熟度,与其它地区不同,研究区石英砂岩具有良好的储集性能,其主要原因是由于石英砂岩中石英颗粒内具有高密度原生微裂缝,这些原生裂缝在沉积后成岩过程中可能闭合,但在后期的构造挤压背景下石英砂岩的脆性特性及破碎颗粒容易导致大量裂缝发育,裂缝的发育同时促进原生裂缝充填物和粒间填屑物的溶蚀,形成以裂缝为主的孔隙性储层。在研究区具有裂缝的石英颗粒并不局限在石英砂岩,我们推测具有裂缝的石英与物源有关。     

Geochemistry and isotope systematics of calc-alkaline volcanic rocks from the Saar-Nahe basin (SW Germany) – implications for Late-Variscan orogenic development

Late Carboniferous (300–290 Ma) calc-alkaline basalts, andesites, and rhyolites typical of volcanic arc settings occur in the intermontane Saar-Nahe basin (SW Germany) within the Variscan orogenic belt. The volcanic rock suite was emplaced under a regime of tensional tectonics during orogenic collapse and its origin has been explained by melting of mantle and crust in the course of limited lithospheric rifting. We report major, trace and rare-earth-element data (REE), and Nd-Pb-Sr-O isotope ratios for a representative sample suite, which are fully consistent with an origin closely related to plate subduction. Major and trace element data define continuous melt differentiation trends from a precursor basaltic magma involving fractional crystallization of olivine, pyroxene, plagioclase, and magnetite typical of magma evolution in a volcanic arc. This finding precludes an origin of the andesitic compositions by mixing of mafic and felsic melts as can be expected in anorogenic settings. The mafic samples have high Mg numbers (Mg# = 65–73), and high Cr (up to 330 ppm) and Ni (up to 200 ppm) contents indicating derivation from a primitive parental melt that was formed in equilibrium with mantle peridotite. We interpret the geochemical characteristics of the near-primary basalts as reflecting their mantle source. The volcanic rocks are characterized by enrichment in the large ion lithophile elements (LILE), negative Nb and Ti, and positive Pb anomalies relative to the neighboring REE, suggesting melting of a subduction-modified mantle. Initial _Nd values of −0.7 to −4.6, Pb, and ⁸⁷Sr/⁸⁶Sr_(t) isotope ratios for mafic and felsic volcanics are similar and indicate partial melting of an isotopically heterogeneous and enriched mantle reservoir. The enrichment in incompatible trace elements and radiogenic isotopes of a precursor depleted mantle may be attributed to addition of an old sedimentary component. The geochemical characteristics of the Saar-Nahe volcanic rocks are distinct from typical post-collisional rock suites and they may be interpreted as geochemical evidence for ongoing plate subduction at the margin of the Variscan orogenic belt not obvious from the regional geologic context. Received: 3 August 1998 / Accepted: 2 January 1999     

对北祁连山火山喷溢型贱金属硫化物矿床区域成矿的探讨

北祁连山加里东优地槽褶皱山系由厚层海相火山岩系和沉积建造组成。其内已经发现数十个火山岩型块状硫化物矿床,可划分为三种类型,即Cu(Fe)型(或红沟型)、Cu-Zn型(或蛇绿岩套型)及Cu-Pb-Zn型(或白艰厂型)。不同类型矿床分别形成于不同地质环境且趋向于在特定时-空范围产出:Cu(Fe)型矿床集中分布在南部达坂山成矿带,成矿与晚奥陶世双峰态细碧角斑岩系有关,矿床形成于弧间或弧后盆地环境;Cu-Zn型矿床分布在北部九个泉—错沟、猪嘴哑吧一银硐沟矿带,与早—中奥陶世蛇绿岩套有关,矿床形成于大洋扩张构造环境;Cu-Pb-Zn型矿床集中分布在祁连、白银成矿区,矿床产出与中寒武世中酸性石英角斑凝灰岩有关,形成于优地槽发展早期阶段裂谷岛弧环境。     

鄂尔多斯盆地陇东地区延长组砂岩成岩作用及孔隙演化

大量岩石薄片、孔隙铸体薄片、扫描电镜与X衍射以及常规物性等分析资料研究表明,鄂尔多斯盆地陇东地区三叠系延长组砂岩主要为成分成熟度及结构成熟度较低的长石质岩屑砂岩、岩屑砂岩和长石砂岩。优质储层主要发育于粗粒富含刚性颗粒的砂岩中,这类砂岩在埋藏过程中由于早期快速埋藏加之泥岩、板岩及千枚岩岩屑含量高,经历了强烈的压实作用,使其由原始孔隙度大约为39%降低至18.5%;此后发生石英颗粒次生加大及绿泥石胶结,使孔隙有一定程度的损失(大约3.5%);紧随其后的是方解石及白云石碳酸盐胶结,损失孔隙度5%;后来出现碳酸盐胶结物与长石及岩屑的溶蚀作用,新增加孔隙度4％～8%,最晚期的是含铁方解石和含铁白云石胶结充注溶蚀孔隙和部分原生孔隙。     

Diagenesis impact on a deeply buried sandstone reservoir (Es1 Member) of the Shahejie Formation,Nanpu Sag,Bohai Bay Basin,East China

Diagenesis has a significant impact on reservoir quality in deeply buried formations. Sandstone units of the Shahejie Formation (Es₁ Member) of Nanpu Sag, Bohai Bay Basin, East China is a typical deeply buried sandstone with large hydrocarbon accumulations. The methodology includes core observations and thin section studies, using fluorescence, scanning electron microscope (SEM), cathodoluminescence (CL), fluid inclusion and isotope and electron probing analysis as well as the numerical determination of reservoir characteristics. The sandstones consist of medium to coarse-grained, slight to moderate sorted lithic arkose and feldspathic litharenite. Porosity and permeability values range from 0.5 to 30% and 0.006 to 7000 mD, respectively. The diagenetic history reveals mixed episodes of diagenesis and deep burial followed by uplift. The main diagenetic events include compaction, cementation alteration, dissolution of unstable minerals and grain fracturing. Compaction resulted in densification and significantly reduced the primary porosity. Quartz, calcite and clay are the dominant pore-occluding cement and occur as euhedral to subhedral crystals. Alteration and dissolution of volcanic lithic fragments and pressure solution of feldspar grains were the key sources of quartz cement whereas carbonate cement is derived from an external source. Clay minerals resulted from the alteration of feldspar and volcanic lithic fragments. Porosity and permeability data predict a good inverse relationship with cementation whereas leaching of metastable grains, dissolution of cement and in some places formation of pore-lining chlorite enhanced the reservoir quality. The best reservoir is thicker sandstone bodies that are medium to coarse-grained, well-sorted sandstone with low primary ductile grains with a minor amount of calcite cement. The present study shows several diagenetic stages in the Es₁ Member, but the overall reservoir quality is preserved.     

Modern sands of South America: composition,provenance and global significance

Standard petrographic methods were used to study 604 modern sands from South America, of which 351 came from rivers and 253 from beaches. In spite of the wide geomorphic contrasts, these sands belong to only three great families: (1) an immature Andean family of lithic arenites rich in volcanic and metamorphic grains that covers about 30% of South America; (2) a cratonic association rich in quartz that covers about 62% of the continent; and (3) a transitional, molasse association, which occurs between these two and covers only about 8% of the continent. The grand arithmetic average Q:F: Rf ratio for the entire continent is 60:11:29 and the area-weighted, carbonate-free average Q_a:F_a:Rf_a is 68:10:22, a superior continental estimate, and the first ever for an entire continent.Rock fragments are the most informative of all the provenance indicators and in South America they range from tropical alterites of the cratonic family through the dominant volcanic and metamorphic grains of the Andean family to biogenic carbonate in beach sands. Carbonate grains are almost totally absent in modern South American rivers because of present day tropical weathering and a long geological evolution that has favored sand production. They are present in many of the beach sands, however, and are most abundant along the tropical Brazilian and Caribbean coasts. Metamorphic grains rival volcanics as earmarks of the Andean family and survive tropical weathering far better than volcanic grains.There is a strong association between continental soil types and the composition of river sand, but the ultimate controls are tectonic history — the ancient landscapes preserved on the Gondwana shields of South America versus the tectonically young landscapes of the Andes — and climate. Climate can either cause leaks to a far distant ocean where weathering is retarded by aridity or it can serve as a barrier to continental sand dispersion, where tropical weathering eliminates unstable grains in transit to the sea.When provenance studies are conducted across an entire, unglaciated continent, events in the far distant past need to be considered when seeking fundamental explanations of its contemporary, surficial sands.     

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。     

Criteria for the identification of ancient volcanic arcs

Characteristic features of recent volcanic arcs must be preserved in the rock record to be useful in determining the magmatic affinities of metavolcanic rocks. This paper reviews criteria suggested by others, and proposes new criteria for the recognition of ancient volcanic arc complexes. Major element abundances, which discriminate magma types in recent volcanic rocks, are very susceptible to modification during low-grade metamorphism, and therefore are of limited value for determining magmatic affinities of altered volcanic rocks. Ti and Zr, Cr and the rare-earth elements, are only slightly affected by low-grade metamorphism. These elements show distinctive trends that allow ocean-floor basalts to be discriminated from most volcanic arc basalts. Clinopyroxene phenocrysts are commonly the only unaltered remnant phase present in metavolcanic rocks. Compositions of clinopyroxene phenocrysts from a suite of fractionated volcanic rocks can be employed as a petrogenetic indicator, because each magma series displays a distinctive CaFeMg trend during differentiation. The much greater abundance of pyroclastic volcanic rocks versus flows in modern volcanic are sequences is a preservable criterion for identifying ancient volcanic arcs. Interbedded with the pyroclastic volcanic rocks are thick deposits of graywackes and mudstones. The volcanic arc deposits are overprinted by high-temperature/low-pressure metamorphism. Parallel to and on the seaward side of the volcanic arc metamorphic belt is a belt of low-temperature/high-pressure metamorphic rocks. These two metamorphic belts comprise a paired metamorphic belt that is diagnostic of Pacific-type convergent plate margins. These criteria together distinguish volcanic arc deposits from other volcanic—sedimentary deposits.     

五宝场沙溪庙组裂缝成因分析

五宝场沙溪庙组裂缝主要是构造缝,包括充填缝、部分充填的张开缝、溶蚀缝,局部裂缝发育处溶洞,洞密度局部0.111个/m～26.67个/m。裂缝成因主要为构造成因。五宝场盆地受来自喜山期起主导作用的来自南东方向的挤压力和其在工区南西方向派生的指向北东的应力、来自北东的挤压力及其派生的应力控制;五宝场沙溪庙组储集岩主要为灰、灰白色长石石英砂岩、长石砂岩、浅灰色长石岩屑砂岩、岩屑长石砂岩及岩屑砂岩等类型,总体上储层砂岩为脆性岩层,具有低胶结物含量,低成份成熟度,高结构成熟度的岩石学特征[8]。     

Sources of sand tempers in prehistoric Tongan pottery

The sources of volcanic temper sands in prehistoric pottery found on low-lying raised-coral islands of Tonga have long been uncertain. The paucity of noncalcareous sand on most inhabited Tongan islands has led to the suggestion that temper was imported from the active magmatic are to the west, or even that most pottery was imported to Tonga from Fiji. The simple mineralogy of the tempers, which are composed almost exclusively of pyroxene and plagioclase mineral grains, volcanic rock fragments, and opaque ferro-magnesian oxide grains, is compatible with derivation from Tongan volcanics, but island volcanoes of Tonga lack deposits of rounded and well-sorted sands that are similar texturally to the tempers. Discovery of local placer deposits composed of volcanic sand on beaches otherwise composed of calcareous reef detritus within the Ha'apai Group of central Tonga provides a satisfactory local source for temper on the shorelines of several inhabited islands. The beach placer sands were apparently derived from reworking of thick tephra deposits that mantle the islands. Detailed compositional analysis of temper sands in numerous ancient Lapitoid potsherds from all the constituent island groups of Tonga implies that pottery making was once a widespread industry making use of locally available raw materials. Stylistically diagnostic protohistoric Fijian potsherds, found in small numbers on several islands in Tonga, can be distinguished from the older indigenous Lapitoid wares by their anomalous tempers, which contrast sharply with the Tongan volcanic temper sands. © 1996 John Wiley & Sons, Inc.     

胶-辽-吉古元古代造山/活动带巨量变沉积岩系的研究进展

华北克拉通发育三条古元古代构造带,包括:东部陆块内部的胶-辽-吉带(Jiao-Liao-Ji belt)、西部陆块内部的孔兹岩带(Khondalite belt)以及两个陆块之间的中部造山带(Trans-North China Orogen)。通过二十多年的深入研究,在区域构造、变质地质、岩浆作用、地球化学、同位素年代学以及地球物理等方面积累了大量资料,并取得了一系列重要的科学进展。其中,胶-辽-吉带是华北克拉通最具代表性的一条古元古代造山/活动带,它不仅接受了古元古代巨量的陆壳物质沉积,而且经历了十分复杂的构造演化过程,并经受了多期岩浆-变质事件的改造。胶-辽-吉造山/活动带的物质组成最为丰富,以大面积分布的巨量(火山)沉积岩系为特征,在中国境内包括吉南地区的集安群和老岭群、辽东南地区的南辽河和北辽河群、胶北地区的荆山群和粉子山群,向南西则有可能穿越郯庐断裂延伸至徐州-蚌埠一带的五河群,总体呈NE向展布,延伸规模长约1000km。从巨量沉积岩系的岩石组合和空间分布特征来看,荆山群与南辽河群、集安群可以对比,而粉子山群则与北辽河群、老岭群相当。然而,由于多期/多阶段强烈构造变形作用的影响,原来各群、组中地层的上下层位及接触关系已完全破坏,目前均已呈规模不一的构造岩片形式叠置在一起,彼此之间呈断层或韧性剪切带接触。巨量变沉积岩系的源区物质主要来源于造山/活动带内古元古代花岗质岩石和两侧古老陆块的变质基底,原岩形成时代为1.95~2.15Ga左右。以往研究表明,胶-辽-吉造山/活动带变质作用的强度十分不均匀,(中-高压)麻粒岩相变质只局限于胶北的荆山群及相关岩石,而粉子山群以及辽东南的南、北辽河群和吉南的集安群、老岭群只经历了角闪岩相变质,局部甚至只达到绿片岩相变质。粉子山群、北辽河群和老岭群变质演化P-T-t轨迹具有顺时针型式,而荆山群、南辽河群和集安群的P-T-t轨迹则具有逆时针型式。本文最新研究发现,古元古代麻粒岩相变质作用并非只局限于胶北地区的荆山群及其邻区,而是贯穿于整个辽东南地区的南辽河群和吉南地区的集安群,其变质演化P-T-t轨迹与胶北地区荆山群泥质麻粒岩以及基性麻粒岩一样,均具有典型近等温减压(ITD)顺时针型式,整个胶-辽-吉造山/活动带的麻粒岩相峰期变质时代为1.9~1.95Ga左右。野外观察和室内岩相学研究表明,在麻粒岩相变质作用过程中,胶北的荆山群及相关岩石、辽东南的南辽河群以及吉南的集安群中的泥质麻粒岩均广泛发生了深熔作用,长英质脉体呈不规则细脉状、网脉状和透镜状分布于寄主岩石中,且与寄主岩石之间呈渐变过渡关系。深熔锆石U-Pb定年结果显示,区域性的深熔作用(或部分熔融)时代为1.84~1.86Ga之间,表明这期广泛的深熔事件应发生于胶-辽-吉造山/活动带整体构造折返的中-低压麻粒岩相退变质阶段。有关胶-辽-吉古元古代造山/活动带的空间展布、南北边界、延伸规律及其形成的大地构造背景一直存在着分歧和争议,最新研究表明,蚌埠-霍邱一带地表露头及其以西第四系覆盖区之下的花岗质岩石、基性麻粒岩和富Al片麻岩岩心,均记录了1.85~1.95Ga的麻粒岩相变质事件,暗示着胶-辽-吉造山/活动带更有可能穿越郯庐断裂,向鲁西南延伸至蚌埠-霍邱一带及其以西的第四系覆盖区之下的变质基底。而辽南地块和狼林地块大量1.85~1.95Ga变质热事件和1.8~1.9Ga、~2.1Ga两期岩浆事件的记录,则表明辽南地块和狼林地块(至少是一部分变质基底)曾卷入到胶-辽-吉古元古代构造演化事件之中。有关胶-辽-吉古元古代造山/活动带构造演化过程及其形成的大地构造背景,目前有三种构造模式,包括:裂谷开启-闭合模式、弧(陆)-陆碰撞模式和先裂谷-后碰撞造山演化模式,然而,带内异常复杂的巨量火山-沉积岩系的物质组成、多期/多阶段的岩浆作用事件、多种变质作用类型和十分复杂的变质演化P-T-t轨迹样式、多期/多阶段复杂的构造变形特征,难以采用上述任何一种构造演化模式来加以合理解释。由此可见,有关胶-辽-吉古元古代构造/活动带南侧边界需要进一步准确厘定,有关狼林地块和辽南地块的构造归属,特别是胶-辽-吉造山/活动带在古元古代构造演化的动力学过程及其形成的大地构造背景还有待进一步深入探讨。     

Zircon and whole-rock Nd-Pb isotopic provenance of Middle and Upper Ordovician siliciclastic rocks, Argentine Precordillera

Graptolite‐bearing Middle and Upper Ordovician siliciclastic facies of the Argentine Precordillera fold‐thrust belt record the disintegration of a long‐lived Cambro‐Mid Ordovician carbonate platform into a series of tectonically partitioned basins. A combination of stratigraphic, petrographic, U‐Pb detrital zircon, and Nd‐Pb whole‐rock isotopic data provide evidence for a variety of clastic sediment sources. Four Upper Ordovician quartzo‐lithic sandstones collected in the eastern and central Precordillera yield complex U‐Pb zircon age spectra dominated by 1·05–1·10 Ga zircons, secondary populations of 1·22, 1·30, and 1·46 Ga, rare 2·2 and 1·8 Ga zircons, and a minor population (<2%) of concordant zircons in the 600–700 Ma range. Archaean‐age grains comprise <1% of all zircons analysed from these rocks. In contrast, a feldspathic arenite from the Middle Ordovician Estancia San Isidro Formation of the central Precordillera has two well‐defined peaks at 1·41 and 1·43 Ga, with no grains in the 600–1200 Ma range and none older than 1·70 Ga. The zircon age spectrum in this unit is similar to that of a Middle Cambrian quartz arenite from the La Laja Formation, suggesting that local basement rocks were a regional source of ca 1·4 Ga detrital zircons in the Precordillera Terrane from the Cambrian onwards. The lack of grains younger than 600 Ma in Upper Ordovician units reinforces petrographic data indicating that Ordovician volcanic arc sources did not supply significant material directly to these sedimentary basins. Nd isotopic data (n = 32) for Middle and Upper Ordovician graptolitic shales from six localities define a poorly mixed signal [ɛ_Nd(450 Ma) = −9·6 to −4·5] that becomes more regionally homogenized in Upper Ordovician rocks (−6·2 ± 1·0; T_DM = 1·51 ± 0·15 Ga; n = 17), a trend reinforced by the U‐Pb detrital zircon data. It is concluded that proximal, recycled orogenic sources dominated the siliciclastic sediment supply for these basins, consistent with rapid unroofing of the Precordillera Terrane platform succession and basement starting in Mid Ordovician time. Common Pb data for Middle and Upper Ordovician shales from the western and eastern Precordillera (n = 15) provide evidence for a minor (<30%) component that was likely derived from a high‐μ (U/Pb) terrane.