Volcanic evolution of the marginal and interarc basins

Volcanic evolution of the interarc and marginal basins is analysed using the available data on volcanics from the presently existent and ancient back-arc basins of the western Pacific and Mediterranean. It is shown that in early (pre-spreading) stages of back-arc rifting, the character of volcanism is determined by “maturity” of the adjacent island arc. It is predominantly alkaline or mildly alkaline for back-arc basins related to the island-arcs with high-potash calc-alkaline and shoshonitic volcanism. The back-arc alkaline and mildly alkaline basalts strongly differ from the continental and oceanic rift volcanoes by constantly lower Ti, Nb and Zr contents. Because of these features these basalts are akin to the basaltic members of the island-arc volcanic series. As the latter, they are generally strongly enriched in K₂O and LIL elements, whereas Na₂O reveals comparatively small variability. With initiation of spreading a sharp depression of K₂O, LIL and light REE occurs in the axial basalts of back-arc basins, that progressively approach the MORB composition. But even tholeiites from the most evolved basins that underwent a considerable spreading reveal slight but detectable geochemical peculiarities, indicating their island-arc affinities. Origin of the low-Ti alkaline basaltic magmas of the active continental margins is discussed.     

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.     

孟加拉湾地区大陆边缘盆地勘探概况与油气富集主控因素

孟加拉湾位于印度大陆以东、缅甸-安达曼-苏门答腊以西、孟加拉国南部海上地区,该区存在主动和被动两种不同类型的大陆边缘,并发育众多大陆边缘含油气盆地。根据板块位置和构造特征将其划分为三大类,分别是:被动大陆边缘盆地(马哈纳迪、K-G和高韦里盆地);主动大陆边缘盆地(若开、缅甸中央、马达班、安达曼和北苏门答腊盆地);残留洋盆地(孟加拉盆地)。根据火山岛弧带分布情况进一步将主动大陆边缘盆地划分为:①海沟型——若开盆地;②弧前型——缅甸中央盆地;③弧后型——马达班、安达曼和北苏门答腊盆地。对这些盆地油气勘探情况的统计与分析表明,该区大陆边缘盆地的油气分布主控因素为:烃源岩类型与有机质丰度决定了流体性质与资源强度;大型河流—三角洲形成富油气区;盆地类型、性质及晚期构造活动强度决定区带勘探潜力。     

Petromineralogic and rock magnetic aspects of clastic sedimentation in the Surma basin, Mizoram

We present here comprehensive petromineralogic, mineral magnetic and lithologic observations from five stratigraphic sections representing the Barail Group, Middle and Upper units of Bhuban Formation, Bokabil Formation and Tipam Group in the Mizoram area. These stratigraphic units mainly display interplay of the grey and buff colored sandstones of the clast compositions varying from sub-lithic to lithic arenites (Q₇₉F₄L₁₇ to Q₅₅F₃L₄₂) including sedimentary, meta-sedimentary and subordinate igneous rock fragments. The buff sandstones with higher lithic fragments [Q₆₉F₄L₂₇ (Ls₆₁Lm₃₈Lv₁)] are dominated by recycled components with higher clast angularity (VA₁A₁₅SA₅₁SR₂₄R₈WR_0.4) relative to the grey sandstones. Mineral magnetic studies decipher bimodal (ferri- and antiferromagnetic) mineralogy with higher concentration in buff sandstones relative to the unimodal ferrimagnetic nature of the grey sandstones. The study infers that the buff sandstones mark the regressive phases driven by hinterland uplifts; whereas the growth of the grey sandstone facies is marked by transgressive basinal processes. Gradual increase in the frequency and appearance of the buff sandstones in the Surma stratigraphy, therefore, can be related to the evolution of the Indo-Burmese ranges.     

西太平洋中段沉积盆地新生代构造-沉积演化特征*

西太平洋中段位于欧亚板块、太平洋板块和印度洋板块的交汇处,是全球沟弧盆体系最为发育的地区,主要发育弧后盆地、弧前盆地和陆架边缘盆地。文中通过综合研究西太平洋中段17个盆地的大地构造背景、盆地的形成与演化和沉积特征后认为:(1)各类盆地的构造-沉积演化均经历4个期次。弧前盆地和弧后盆地的构造-沉积演化可分为俯冲期(K₂-E)、盆地发育期((Ｎ_１^１ -Ｎ_１³)、沉降期(Ｎ_１⁴-Ｎ_１⁵)和挤压期(N₂-Q):俯冲期发育火山岩和变质岩基底,局部为海相碎屑岩;盆地发育期以海相沉积为主,伴有火山活动,局部发育陆相沉积;沉降期以海相和三角洲相沉积为主;挤压期以三角洲相和海相沉积为主,局部发育河湖相沉积。陆架边缘盆地的构造-沉积演化也分为4期,分别为前裂陷期(K₂-E₁)、裂陷期(E₂-(E₃^１)、拗陷期(E₃²-Ｎ_１³)和沉降期(Ｎ_１⁴-Q):前裂陷期和裂陷期主要发育冲积扇—河流—湖泊沉积体系,火山活动强烈;拗陷期沉积环境由陆相向海陆过渡相演化;沉降期以海相和海陆过渡相沉积为主。(2)不同类型盆地的构造-沉积演化特征各不相同:弧前盆地构造以挤压和板块俯冲为主,平均沉积厚度为3.6,km,总体由海陆过渡相向陆相演化;弧后盆地构造受板块俯冲后撤和弧后洋壳扩张作用控制,平均沉积厚度为4.8,km,总体由海相向海陆过渡相演化;陆架边缘盆地构造呈下断上拗双层结构,平均沉积厚度超过10,km,总体由陆相演化为海相。     

阿尔金南缘断裂带球状辉长岩的发现及意义

本文报道了在新疆阿尔金山南缘断裂带中首次发现的球状辉长岩，及其产出特征与岩石学、地球化学特征。提出球状辉长岩代表了弧后盆地或活动大陆边缘位置，来自消亡洋壳和上覆楔状地幔岩这两种不同源岩组份的混合物。球状辉长岩的岩浆由消亡洋壳熔融直接产生，但熔体在上升达到地表的过程中，与楔状地幔岩发生反应，并加速楔状地幔岩的熔融，同时发生结晶作用和分异作用。     

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.     

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.     

East Sakhalin island arc paleosystem of the Sea of Okhotsk region

It has been established that volcanic rocks of the Schmidt, Rymnik, and Terpeniya terranes are fragments of the compound Early to Late Cretaceous-Paleogene East Sakhalin island arc system of the Sea of Okhotsk region. This island arc paleosystem was composed of back-arc volcano-plutonic belt, frontal volcanic island arc, fore-arc, inter-arc, and back-arc basins, and the Sakhalin marginal paleobasin. The continental volcanic rocks dominate in the back-arc volcano-plutonic belt and frontal volcanic island arc. The petrochemical composition of basalts, basaltic andesites, andesites, and trachytes from the frontal island arc formed in submarine conditions are typical of oceanic island arc or marginal sea rocks (IAB). The petrochemical composition of volcanic rocks from the island arc structures indicates its formation on the heterogeneous basement including the continental and oceanic blocks.     

Petrological features of a Numidian section in the Lucanian Apennine (southern Italy)

A continuous section of Numidian sandstones cropping out along the eastern margin of the Lucanian Apennine (southern Italy) has been studied in relation to its petrographic and geochemical aspects. Owing to their simple mineralogy, both chemical and petrographic classifications indicate that the sandstones are subarkoses. They are characterized by high mineralogical maturity, poor sorting, abundant siliciclastic matrix and subangular quartz grains. The principal composition is Q_80–94 F_6–20 L_0–1 corresponding to SiO₂/Al₂O₃ and Fe₂O₃/K₂O ratios ranging from 37·216 to 17·124 and from 1·182 to 0·514 respectively. Two quartz grain types were distinguished: deformed and undeformed quartz grains, the former generally subangular in shape and the latter rounded. Two distinct origins of detritus can be suggested; (i) the undeformed and rounded quartz grains derive from arenaceous continental sequences; (ii) the siliciclastic matrix, the deformed subangular quartz grains, the heavy minerals and the large crystals of K-feldspar probably derive from a crystalline basement formed by low–medium–high grade metamorphic rocks and granitoids. The Numidian subarkoses represent the mixing of these two components. Both kinds of detritus can be referred to the African Craton where crystalline basement is extensively overlain by Palaeozoic and Mesozoic sandstones. The different proportions of the two components in the mixture account for the variability of the sandstone composition and might explain the mixed Numidian successions cropping out in various sectors of the Mediterranean area. Copyright © 1998 John Wiley & Sons, Ltd.     

Petrography and geochemistry of the Upper Cretaceous Nubia sandstones,Eastern Desert,Egypt: implications for provenance,paleoweathering, and tectonic setting

The provenance of the Upper Cretaceous Nubia sandstones from four vertical sections along Qena-Safaga and Qena-Quseir roads in central Eastern Desert of Egypt was investigated based on their modal composition and geochemical data. The Nubia sandstone samples are abundant in quartz content with low feldspar and lithic fragments. Their average modal composition (Q_94.2F_1.3R_4.5) classifies them as quartz arenites with subordinate quartz wackes which is consistent with geochemistry data. The average CIA, CIW, PIA, and Th/U ratio values revealed that the intensity of weathering in the studied areas was similar, varying from moderate to intensive weathering, and may reflect low-relief and warm humid climatic conditions in the source area. The ICV (<?1) and SiO₂/Al₂O₃ ratio (>?5) indicate that the Nubia sandstones are texturally and mineralogically mature. The petrographical and geochemical analyses suggest that the Nubia sandstones were mainly derived from felsic (granitic) and/or recycled sand sources. The major element-based multidimensional tectonic discrimination diagrams suggested the Nubia sandstones were deposited in a passive continental margin of a syn-rift basin. This result agreed with the general geology of central Eastern Desert of Egypt during the Upper Cretaceous.     

Continental margins as global oil and gas accumulation belts: The West Pacific belt

Most present-day oil- and gas-bearing (petroliferous) basins are localized in one of the five global oil and gas accumulation belts confined to continent-ocean transition zones that existed in the Mesozoic and Cenozoic. Two meridional belts are located in the western and eastern peripheral zones of the Pacific Ocean with intense tectonic activity during the major part of the Mesozoic and Cenozoic. The activity was reflected in extension of the continental crust, spreading of the oceanic crust, rapid subsidence of individual crustal blocks, volcanism, and formation of large batholiths and accretionary prisms. In this belt, the fore-arc, back-arc, inter-arc, and marginal riftogenic sedimentary basins are petroliferous.     

Composition of plagioclase feldspar in modern deep-sea sands: relationship to tectonic setting

Microprobe analysis of plagioclase in deep-sea sands from various tectonic settings gives averages ranging from 19% An for trailing-edge continental margins to 56% An for basins on the ocean side of an island arc. For each setting except island arcs, anomalously high amounts of pure albite are found. These albitic grains may be recycled from pre-existing sedimentary rocks. Potassium-rich plagioclase, thought to be indicative of a volcanic provenance, is less abundant than would be expected from the proportion of volcanic rock fragments. Perhaps such high-K plagioclases are less resistant to weathering than low-K plagioclase.     

Geological structure,sedimentation conditions,and petroleum potential of sedimentary basins in Southeast Asia

Sedimentary basins located in the continent-to-ocean transition zones with an active tectonic regime are extremely different in terms of both geological structure and composition of rocks with the hydrocarbon-hosting potential. Oil-and-gas fields in such zones are confined not only to the deltaic, coastal-marine, and submarine-slope sandstones, as well as limestones and dolomites of buried carbonate banks, but also to silicic and volcanic rocks. Such fields have been discovered in weathering crusts of granite plutons, metamorphic rocks of the basement, fissured basalts, and other magmatic rocks. This is demonstrated vividly by the Southeast Asia region, including Indonesia, Malaysia, Vietnam, Bangladesh, and other countries, where fore-arc, back-arc, inter-arc, and marginal-rift sedimentary basins are assigned to petroliferous areas.     

Integrated petrographic,mineralogical, and geochemical study of the upper Kaimur Group of rocks,Son Valley,India: Implications for provenance,source area weathering and tectonic setting

The upper Kaimur Group (UKG) of the Vindhyan Supergroup in central India, primarily consists of three rock types-Dhandraul sandstone, Scarp sandstone and Bijaigarh shale. The present study aims to reconstruct the parent rock assemblages, their tectonic provenance, mineralogy, weathering intensity, hydraulic sorting and depositional tectonic setting. Samples from the UKG rocks representing the Dhandraul sandstone, Scarp sandstone and Bijaigarh shale were studied using a combination of petrographic, mineralogical, and geochemical techniques. Texturally, medium to coarse grained UKG sandstones are mature and moderate to well sorted. Deficiency of feldspars in these sandstones indicates that the rocks are extensively recycled from distant sources. Their average modal composition for Scarp (avg. Qt₉₉ F₀.₂L_0.8) and Dhandraul (avg. Qt₉₉ F_0.1L_0.8) sandstones, classifies them as quartz arenite to sub-litharenite types, which is consistent with geochemical study. Major element concentrations revealed that sandstones have high SiO₂, K₂O < Na₂O, and low Fe₂O₃, which are supported by the modal data. On the other hand, sandstone samples are enriched in most trace elements such as Ce, Sr, V, Sc and Zr and depleted in U and Th. The CIA values (43.17–76.48) of the UKG rocks indicate low to moderate weathering, either of the original source or during transport before deposition, which may have related to low-relief and humid climatic conditions in the source area. Further, petrographic and geochemical interpretations indicate that they are derived from craton interior to quartzose recycled sedimentary rocks and deposited in a passive continental margin. Therefore, granitic and low grade metamorphic rocks of Mahakoshal Group and Chotanagpur granite-gneiss, situated on the southern and south-eastern side of the Vindhyan basin are suggested as possible provenance for the UKG rocks.     

http://www.sciencedirect.com/science/article/pii/S1674987112001533

The collision of a divergent ocean ridge may evolve into two end cases:in the continuity of ocean-floor subduction.or in the detachment of the subducted plate.The northern Patagonia active plate margin has the unique situation that in Cenozoic time it has been subjected to two divergent ridge collisions,each one representing one of the end members.The Neogene Antarctica-Nazca divergent ridge collision evolved as a continuous ocean-floor subduction system,promoting a magmatic hiatus at the arc axis,the obduction of part of the ridge ocean-floor in the fore-arc.and basaltic volcanism in the back-arc.In contrast,the Paleogene Farallon-Aluk divergent ridge collision evolved into a transform margin,with the detachment and sinking of the Aluk plate and the development of a large slab window.As in the previous case,this collision promoted a magmatic hiatus at the arc axis,but the tectono-magmatic scenario changed to postorogenic synextensional volcanism that spread to the former fore-arc（basalt,andesite,rhyolite） and former back-arc（bimodal ignimbrite flare-up,basalt）.Geochemistry of this slab window synextensional volcanism shows more MORB-like basalts towards the former fore-arc,and MORB-OIB-like basalts towards the former back-arc.Instead,an isolated undeformable crustal block in the former back-arc,with an "epeirogenic" response to the slab window and extensional regime,was covered by OIB-type basalts after uplift.Major elements show that slab window basalts reach TiCh values up to 3 wt%,as compared with the top value of 1.5 wt%of arc magmas.Besides,the MgO with respect to（FeO^t + Al₂O₃） ratio helps to distinguish slab window magma changes from the former fore-arc to the former back-arc and also with respect to the "epeirogenic" block.Higher contents of HFS elements such as Nb and Ta also help to distinguish this slab window from arc magmas and also,to distinguish slab window magma changes from the former fore-arc to the former back-arc and "epeirogenic" block settings.The isotope compositions of slab window magmatism show a disparate coeval array from MORB to crustal sources,interpreted as a consequence of the lack of protracted storage and homogenization due to the extensional setting.     

Geochemistry and Provenance of Maastrichtian Clastic Rocks in the Dikmendede Formation of Orhaniye in Kazan-Ankara-Turkey Region

An integrated petrographic and geochemical study of the sandstones of the Maastrichtian-aged in the Orhaniye (Kazan-Ankara-Turkey) was carried out to obtain more information on their provenance, sedimentological history and tectonic setting. Depending on their matrix and mineralogical content, the Maastrichtian sandstones are identified as lithic arenite/wacke. The Dikmendede sandstones derived from types of provenances, the recycled orogen and recycled transitional. The chemical characteristics of the Dikmendede sandstones, i.e., fairly uniform compositions, high Th/U ratios (>3.0), negative Eu anomalies (Eu/Eu* 0.72–0.99) and Th/Sc ratios (mostly less than 1.0), favor the OUC (old upper continental crust) provenance for the Dikmendede sandstones. The SiO2/Al2O3, Th/Sc (mostly <1.0) and La/Sc (<4.0) ratios are; however, slightly lower than typical OUC, and these ratios may suggest a minor contribution of young arc-derived material. The rare earth element (REE) pattern, and La/Sc versus Th/Co plot suggests that these sediments were mainly derived from felsic source rocks. The Dikmendede sandstones have high Cr (123–294 ppm) and Ni (52–212 ppm) concentrations, Cr/Ni ratio of 1.93, and a medium correlation coefficient between Cr and Ni and corresponding medium to high correlation of both (Cr and Ni, respectively) elements with Co. These relationships indicate a significant contribution of detritus from ophiolitic rocks. As rare earth element data are available for the Dikmendede sandstones, the Eu/Eu* is compared with LaN/YbN. Samples plot in the area of overlapping between continental collision, strike-slip and continental arc basins. The predominantly felsic composition of the Dikmendede sandstones is supported by the REE plots, which show enriched light REE, negative Eu anomaly and flat or uniform heavy REE. The Dikmendede sandstones have compositions similar to those of the average upper continental crust and post-Archean Australian shales. This feature indicates that the sediments were derived mainly from the upper continental crust. The Dikmendede sandstones have chemical index of alteration (CIA) values of 28–49, with an average of 40 indicating a low degree of chemical weathering in the source area. The compositional immaturity of the analyzed sandstone samples is typical of subduction-related environments, and their SiO2/Al2O3 and K2O/Na2O ratios and Co, Sc, Th and Zr contents reflect their oceanic and continental-arc settings. The Dikmendede sandstones were developed as flysch deposits derived from mixed provenance in a collision belt.     

The ancient continental margins of the North American and South American plates and regularities in the occurrence of oil and gas accumulations in them

Various stages of the development of sedimentary basins along the ancient margins of the North American and South American plates are considered. It is shown that the potential of the oil-and-gas bearing is related to a certain stage of evolution of the basins. For the margins of the North American plate, it is the first stage of development in the structure of the ancient Paleozoic continental margins that developed under passive tectonic conditions. For the basins along the ancient margins of the South American plate, it is the second stage, which is the stage of the formation and development of foredeeps overlaid on the earlier structures. An interesting regularity is displayed: than younger the folding-mountain structures that originated in the distal parts of the continental margins, than greater the age range of source rocks in the sedimentary basins preserved there.     

Sedimentation,tectonism and volcanism relationships in two Andean basins of southern Peru

The intermontane basins of the Central Andes, formed from the first episode of the andine tectonic (peruvian phase, Santonian), are characterized by a clastic continental sedimentation, settled in a compressive tectonic context and containing volcanic intercalations. The two basins presented in this paper, have distinct ages and a different geodynamic context, each:

1. The Cuzco-Sicuam Basin, Maestrichtian-Paleocene in age, is a back-arc basin. A coarse grained red bed sedimentation appeared at the same time as the synsedimentary tectonic and volcanic activity increased in the source area.
2. The Moquegua Basin, Oligocene in age, is a fore-arc basin. After a compressive phase, the northeastern border of the basin shows high mobility, characterized by superposed alluvial fans. Within the basin, the coarse grained sediments are associated with volcanic tuffs.

In the two basins, the tectonism, the volcanism and the coarse grained sedimentation occured simultaneously. The coarse grained sedimentation does not appear to be linked with climatic changes, but with the increasing mobility of the source area. The relationship between tectonic regime and volcanic activity in the basin and in the source area is considered.     

Petrography of modern marine sands from the Peru-Chile Trench and adjacent areas†

Sand samples from piston cores taken in the Peru-Chile Trench and adjacent areas were examined petrographically in an attempt to characterize sands from the oceanward side of a continent-margin volcanic arc. Framework composition averaged Q₁₈F₄₄L₃₈, close to the average reported for this type of setting, but showed wide variation in the ratio of feldspar to lithic fragments; quartz was uniformly low, less than 30%. Samples rich in lithic grains were from sites that lie off areas of recent volcanism on the adjacent continent. Samples from off southern Central America had compositions more like those found in island-arc settings, averaging Q₃F₂₁L₇₆, whereas those from the Straits of Magellan increased in quartz and decreased in lithic fragments toward the Atlantic. Plagioclase was greatly predominant over K-feldspar in all samples, and averaged An₂₈. A considerable portion of the plagioclase was close to end-member albite, suggesting recycling from pre-existing sedimentary rocks. Climate had no obvious effect on the framework proportions or the feldspar composition, but the volume of sediment in the trench increases with increasing rainfall on the continent.