Mesozoic radiolarian chert from the middle sector of the Yarlung Zangbo suture zone,Tibet and its tectonic implications

The middle sector of the Yarlung Zangbo suture zone stretches over 200 km long from Ngamring through Geding to Rinbung, roughly along Yarlung Zangbo River valley (Fig. 1). This belt resulted from the closure of the Tethyan ocean and the collision be- tween Indian plate and Lhasa block[1―8]. Lots of works demonstrated that rifting of the Tethyan basin in southern Tibet started from Triassic time. Initial oce- anic crust appeared in the Late Jurassic, and then ex- perienced a rapid sprea…     

Sedimentary geochemistry of the Cambrian-Ordovician cherts: Implication on archipelagic ocean of North Qilian orogenic belt

The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.     

Geochemistry of the Late Paleozoic radiolarian cherts within the NE Jiangxi ophiolite melange and its tectonic significance

Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO₂ (78.40%-89.28%) and high Al₂O₃ (3.42%-11.02%). Low Si/Al ratios (6.3-23) and tight negative correlation between Si/Al and Al₂O₃ of the samples indicate that they are muddy cherts containing high and variable contents of pelitic detritus. Geochemically, they are characterized by Al₂O3/(Al2O3+Fe₂O3) = 0.51-0.90, shale-normalized La_n/Ce_n = 0.76-1.11, Ce/Ce* = 0.91-1.22, V<20_μg/g, V/Y<2.6 and Ti/V>40, resembling those of cherts formed in the continental margin regimes. It is therefore concluded that these late Paleozoic radiolarian muddy cherts were most likely formed in a continental margin regime, and not genetically related to the ophiolite suite in NE Jiangxi. It is also unlikely that an oceanic basin existed between the Yangtze and Cathaysia blocks during the late Paleozoic.     

Geological setting of basaltic rocks in an accretionary complex,Khangai–Khentei Belt,Mongolia

We describe the mode of occurrence and geochemical characteristics of basalts, in the Khangai–Khentei belt in Mongolia, overlain by Middle Paleozoic radiolarian chert in an extensive accretionary complex. These basalts are greatly enriched in K, Ti, Fe, P, Rb, Ba, Th, and Nb in comparison to the composition of the mid‐ocean ridge basalts, indicative of within‐plate alkaline type. Ti/Y vs Nb/Y and MnO/TiO₂/P₂O₅ ratios of the basalts also suggest within‐plate affinities. Considering the geochemical characteristics as well as the conformable relationship with the overlying radiolarian chert, the alkaline basalts were clearly not continental but formed a pelagic oceanic island. The mode of occurrence and geochemistry of the basalts show that the alkaline basaltic volcanic activity had taken place to form an oceanic island in the Paleozoic pelagic region sufficiently far from continents to allow radiolarian ooze accumulation.     

Geochemistry of the Mesozoic bedded cherts of Central Baja California (Vizcaino-Cedros-San Benito): implications for paleogeographic reconstruction of an old oceanic basin

In central Baja California (Vizcaino Peninsula, and Cedros and San Benito Islands) two distinct radiolarian bedded chert sequences of late Triassic and late Jurassic/lowermost Cretaceous age, can be differentiated on lithostratigraphic and geochemical criteria.These bedded chert sequences are part of the conformable sedimentary cover of more or less dismembered ophiolites, which are overthrusted by the San Andrès-Cedros volcanic arc system of middle late Jurassic age.Major and trace elements permit paleogeographic zonation of the late Jurassic/lowermost Cretaceous radiolarites lying conformably upon ophiolites considered as fragments of an oceanic basin floor which developed westward of the San Andrès volcanic arc. Progressive accretion of this oceanic basin floor, along the continental margin is supported by the fact that the more distal radiolarian chert sequences belong to the lowermost structural units of this area.     

Long-lived Paleotethyan pelagic remnant inside Shan-Thai Block: Evidence from radiolarian biostratigraphy

The Shan-Thai Block, regarded traditionally as awhole geotectonic unit by the geologists engaged inthe study of geotectonic evolution of Southeast Asia, issituated to the west of the Ailaoshan and Nan-UttaraditSutures and to the east of the Shan Boundary Faults,and covers southwestern Yunnan, eastern Myanmar,most of Thailand, northwestern Laos, western Malay-sia, and Sumatra[1,2] (fig. 1). However, recent researchshows that it consists of two continental terranes fromGondwana and Cathay…     

Geochemical characteristics of pelagic chert sequences across the Permian-Triassic boundary in southwest Japan

Abstract Geochemical characteristics, mainly of major and trace elements and REE (rare earth elements) of bedded chert and shale/mudstone sequences, across the Permian/Triassic boundary in southwest Japan are examined. The boundary is characterized by the disappearance of bedded cherts, and the interval between the Upper Permian cherts and Lower Triassic (probably Smithian) cherts comprises siliceous shales and organic black mudstones. Bedded cherts are characterized by a gradual depletion of chemical elements from Middle to Upper Permian. However, overlying siliceous shales exhibit a clear enrichment in some elements, especially alkaline metals (such as K, Rb and Cs) and Ti, Th, Y, P₂O₅, and REE in comparison with elements of the PAAS (post Archean Australian shales). This indicates that average components of the upper continental crust were transported in the boundary interval, possibly caused by volcanic activity. Ce-negative shifting in NASC (North American Shales Composite)-normalized REE patterns is characteristic of this interval, and could be related to the deposition of siliceous rocks in Ce-depleted seawater. This was probably caused by an invasion of water mass with a Ce-negative anomaly into the previously existing water mass of the Paleo-Tethys. Weak negative Eu-anomalies in this interval are suggestive of plagioclase fractionation caused by acid volcanisms and the LREE/HREE ratios in the interval show a slightly light-REE enrichment. Organic black mudstones are characteristically intercalated in the interval. These rocks are usually regarded as a product of oceanic deterioration, but in pelagic conditions, organic materials were formed by high primary production that resulted from the active upwelling of ocean floor water currents with rich nutrients. This may have been caused by the inferred mixing of water masses of the Paleo-Tethys and of the Panthalassa in Early Triassic time which was regarded as an event synchronous with an increase in volcanic activity on highly matured island arcs and/or continents.     

Geochemistry of the Middle to Late Permian limestones from the marginal zone of an isolated platform (Laibin, South China)

Major, trace and rare earth elements were measured in 27 samples of the Middle to Late Permian limestones from the Tieqiao section located on the marginal zone of an isolated platform (Laibin, South China). Shale-normalized REE+Y patterns of all samples show notably negative Ce anomalies (0.21–0.66, average 0.33), slightly positive Gd anomalies (1.08–1.30, average 1.20), and positive Y anomalies with superchondritic Y/Ho ratios (36–91, average 55), which are consistent with those of modern shallow seawater. Their relative LREEs enrichment with higher Nd_SN/Yb_SN ratios (0.58–1.80) than those of modern shallow seawater (0.21–0.50) suggests complicated sources of REEs for all samples. Compared with geochemical features of sediments influenced by terrigenous particles and hydrothermal fluids, it is concluded that ambient shallow seawater was the primary source of REEs in these limestones. Comparing the indicators of REE+Y elements (ΣREE, Nd_SN/Yb_SN, Ce/Ce*, Gd/Gd*, Eu/Eu* and Y/Ho) in limestones with those in bedded cherts from the Tieqiao section, we consider that limestone and bedded chert have similar sources of REE+Y elements: ambient shallow seawater with more or less hydrothermal fluids. In addition, there is a completely negative correlation between CaCO₃ and SiO₂ contents in limestones and bedded cherts. These results imply that precipitation of CaCO₃ was inhibited by that of SiO₂ which was derived mainly from hydrothermal fluid, especially in bedded cherts from the Tieqiao section.     

Geochemistry of ophiolite cherts from the Qinling orogenic belt and implications for their tectonic settings

Since the 1980s, one of the important progresses in the study of the Qinling orogenic belt is marked by findings of numerous ophiolite zones[1—4]. On the basis of the former orogenic models of the Paleozoic colli-sional orogeny[1,5,6] and the Mesozoic collision[7—9], another orogenic evolution model from the Paleozoic subduction-collision along the Shangdan suture to the Mesozoic final collision orogeny along the Mianle suture[3,10], including the relicts of the Jining orogeny, has been pr…     

Ladinian radiolarian fauna,siliceous rock from the Xianshuihe Belt,West Sichuan and their tectonic significance

Ladinian radiolarian fauna, including Muelleritortis, Baumgartneria, Oertlispongus, Paroertlispongus, Pseudoertlispongus, etc., was discovered from the siliceous rock of the Runiange Formation in the Xianshuihe belt, West Sichuan Province. Geochemical test on five samples from the siliceous rock indicates that SiO2 content varies in 71.16%-90.06% and Si/Al ratio, in 49-71, which shows that the siliceous rock contains more terrigenous mud sediments. The siliceous rock is characterized by the large ratios of Al2O3/(Al2O3+Fe2O3) (0.63-0.81) and Ti/V (>26), the low ratio of V/Y (<2.8), and low vanadium content (<23 μg/g), which are similar to the geochemical characteristics of continental margin siliceous rock. The Ce/Ce* ratios of the four samples vary in 1.02-1.47 and the LaN/CeN ratio, in 0.75-1.07, which imply that the siliceous rock was deposited in the continental margin basin. But only one sample is similar to the oceanic siliceous rock in REE. Turbidite-siliceous rock bearing radiolarian-basalt asse     

Petrochemistry and origin of pleistocene volcanic rocks from northern Taiwan

The Pleistocene volcanic rocks from northern Taiwan include the Tatun volcano group and the Chilung volcano group. Three rock types occur in this area: Tatun volcano group yield high-alumina basalt and andesites, whereas the chilung volcano group mainly consists of dacites. In addition, amphibole-rich nodules have also been found in different cruptive units of the former volcano group. Around seventy sample of various rock types have been conducted for geochemical studies, including analyses of major elements and trace elements such as Co, Cr, Cu, Li, Ni, Zn, Zr, V, Rb and Sr. Results of Al₂O₃, MnO, TiO₂ total alkali content, MgO/ΣFeO and K₂O/Na₂O ratios and AMF diagram indicate that these Pleistocene volcanic rocks belong to typical calalkaline rock series. Detailed study of the trace elements reveals that these volcanic rocks are closely correlated with rocks of continental margin type with respect to Rb, Cu, Co, Ni, V and Cr contents, and K/Rb and Ni/Co ratios. These rocks are most probably derived from the fractionation of basaltic magma controlled mainly by the crystallization of amphibole and plagioclase with magnetite playing a minor role.     

Lithostratigraphy of Permian marine sequences, Khao Pun Area, central Thailand: Paleoenvironments and tectonic history

Geologic mapping and subsurface lithostratigraphic investigations were carried out in the Khao Pun area (4 km²), central Thailand. More than 250 hand specimens, 70 rock slabs, and 70 thin sections were studied in conjunction with geochemical data in order to elucidate paleoenvironments and tectonic setting of the Permian marine sedimentary sequences. This sedimentary succession (2485 m thick) was re‐accessed and re‐grouped into three lithostratigraphic units, namely, in ascending order, the Phu Phe, Khao Sung and Khao Pun Formations. The Lower to lower Upper Permian sedimentary facies indicated the transgressive/regressive succession of shelf sea/platform environment to pelagic or abyssal environment below the carbonate compensation depth. The sedimentological and paleontological aspects, together with petrochemical and lithological points of view, reveal that the oldest unit might indicate an Early Permian sheltered shallow or lagoonal environment. Then the depositional basin became deeper, as suggested by the prolonged occurrence of bedded chert‐limestone intercalation with the local exposure of shallower carbonate build‐up. Following this, the depositional environment changed to pelagic deposition, as indicated by laminated radiolarian (e.g. Follicucullus sp.) cherts. This cryptic evidence might indicate the abyssal environment during middle Middle to early Late Permian; whereas, previous studies advocated shelf‐facies environments. Following this, the depositional condition might be a major regression on the microcontinent close to Indochina, from the minor transgressive/regressive cycles that developed within a skeletal barrier, and through the lagoon with limited circulational and anaerobic conditions, on to the tidal flat to the sheltered lagoon without effective land‐derived sediments.     

Transverse geochemical variations across the Antarctic Peninsula: Implications for the genesis of calc-alkaline magmas

Magnetic activity throughout the Antarctic Peninsula and the South Shetland Islands has been dominantly of a calc-alkaline nature for the last 200 Ma. Chemically, the plutonic and volcanic products are typical of a continental margin magmatic arc, similar to those from western South America. Within any one area, volcanic and plutonic rocks are compositionally indistinguishable, and all magmatic products show increasing SiO₂, and increasing K/Si, Rb/Si, Th/Si and to a lesser extent Ce/Si and La/Si ratios away from the proposed trench axis. The calc-alkaline basaltic compositions also have high large ion lithophile (LIL; e.g. K, Rb, Th)/high field strength (HFS; e.g. Zr, Nb, Ti) ratios relative to non-orogenic counterparts, and increasing LIL/HFS element ratios with increasing fractionation. It is proposed that the high LIL/HFS element ratios in basaltic and andesitic melts are primary features due to dehydration processes with the subducted slab and to fractionation of minor mineral phases from the melt. The increasing LIL/HFS element ratios in more acid rocks are probably due to removal of minor mineral phases from the melt. Although zone refining may contribute to the spatial variations across the peninsula, we have proposed that an enriched subcontinental mantle provides a viable alternative source for the observed K-h variations and for the increased LIL-element contents found in continental margin calc-alkaline magmas.     

Metallogenesis on a Mesozoic passive continental margin,Antalya Complex,southwest Turkey

A variety of Fe, Mn and trace-metal-enriched Mesozoic pelagic sediments are associated with the tectonically emplaced Antalya Complex in southwestern Turkey. Palaeotectonic settings represented within the complex comprise a continental platform, passing laterally through a Mesozoic passive margin into a zone of marginal oceanic crust, formed during the early stages of continental separation. The origins of the metalliferous sediments are elucidated using mineralogical, major, trace element and REE data, and comparisons with oceanic and ophiolite-related sediments.Late Triassic deposition during the initial continental separation was mostly terrigenous, including detrital carbonate derived from adjacent reef complexes. During the Jurassic and Early Cretaceous the passive margin underwent accumulation of fine-grained terrigenous matter and biogenic silica in deep water below the carbonate compensation depth. Argillaceous mudstones deposited during a regional hiatus at the end of the Upper Triassic show unusual Fe and trace metal enrichment, together with a marked positive Ce anomaly, indicative of slow hydrogenous accumulation.The marginal oceanic crustal zone also shows dominantly terrigenous and siliceous biogenic deposition but with the addition of an important hydrothermal component represented by Fe-Mn deposits. These occur within and immediately above the Upper Triassic lavas of the oceanic crust and as intercalations in the overlying Lower Cretaceous radiolarian chert sequence. Most of these sediments show strong Fe-Mn fractionation; several show a negative Ce anomaly implying rapid incorporation of the REEs from seawater.The Upper Triassic Fe-Mn deposits associated with the lavas are relatively trace-element-depleted and record rapid localised precipitation from relatively high-temperature hydrothermal solutions. By contrast, the more manganiferous and trace-element-enriched metalliferous horizons in the Jurassic to Lower Cretaceous chert sequences represent more dilute low-temperature hydrothermal discharge. Regional comparisons suggest that dominantly manganiferous deposits free of sulphides are characteristic of the early formed Mesozoic ocean crust compared with well established spreading axes like the Troodos Massif, Cyprus.     

Off-scraped Permian-Jurassic bedded chert thrust on Jurassic-early Cretaceous accretionary prism: Radiolarian evidence from le Island, central Ryukyu Island Arc

Abstract The pre-Neogene basement of the central Ryukyu Island Arc shows zonal structures analogous to those of the outer belt of southwest Japan. The innermost terrane (Iheya Zone) consists of isoclinally folded beds dipping northwestward; the anticlinal cores are composed mainly of Permian chert, whereas the synclinal parts are represented by Jurassic to Cretaceous sandstone-rich alternating siliceous shale and chert, bearing appropriate radiolarian fossils. At the east-central area of Ie Island, the basement rocks are exposed as a 172 m high peak, Tattyu. The flank area of Tattyu is composed of latest Jurassic to Berriasian siliceous shale and chert as part of an accretionary prism, while most of Tattyu is composed of a continuous and very compact sequence of Norian through Kimmeridgian (?) bedded chert which is rather gently inclined. Beyond an unexposed part below the Norian chert, Guadalupian chert is recognized. It is inferred that this pelagic chert (Tattyu sequence) was off-scraped and thrust on to the accretionary prism which developed on its flank area in an accretion process after the Early Cretaceous.     

Combining machine learning techniques,microanalyses and large geochemical datasets for tephrochronological studies in complex volcanic areas: New age constraints for the Pleistocene magmatism of central Italy

Characterization, correlation and provenance determination of tephra samples in sedimentary sections (tephrochronological studies) are powerful tools for establishing ages of depositional events, volcanic eruptions, and tephra dispersion. Despite the large literature and the advancements in this research field, the univocal attribution of tephra deposits to specific volcanic sources remains too often elusive. In this contribution, we test the application of a machine learning technique named Support Vector Machine to attempt shedding new light upon tephra deposits related to one of the most complex and debated volcanic regions on Earth: the Pliocene-Pleistocene magmatism in Italy. The machine learning algorithm was trained using one of the most comprehensive global petrological databases (GEOROC); 17 chemical elements including major (SiO₂, TiO₂, Al₂O₃, Fe₂O_3T, CaO, MgO, MnO, Na₂O, K₂O, P₂O₅) and selected trace (Sr, Ba, Rb, Zr, Nb, La, Ce) elements were chosen as input parameters. We first show the ability of support vector machines in discriminating among different Pliocene-Pleistocene volcanic provinces in Italy and then apply the same methodology to determine the volcanic source of tephra samples occurring in the Caio outcrop, an Early Pleistocene sedimentary section located in Central Italy. Our results show that: 1) support vector machines can successfully resolve high-dimensional tephrochronological problems overcoming the intrinsic limitation of two- and three-dimensional discrimination diagrams; 2) support vector machines can discriminate among different volcanic provinces in complex magmatic regions; 3) in the specific case study, support vector machines indicate that the most probable source for the investigated tephra samples is the so-called Roman Magmatic Province. These results have strong geochronological and geodynamical implications suggesting new age constraints (1.4 Ma instead of 0.8 Ma) for the starting of the volcanic activity in the Roman Magmatic Province.     

MnO/TiO2/P2O5: a minor element discriminant for basaltic rocks of oceanic environments and its implications for petrogenesis

A ternary diagram using MnO, TiO₂, P₂O₅ can discriminate between five petrotectonic environments of basaltic rocks (45–54% SiO₂). Fields for mid-ocean ridge, island arc tholeiite, island arc calc-alkaline, ocean island tholeiite, and ocean island alkalic rocks were distinguished on the basis of 507 analyses from well-defined environments. Boninites plot within island arc fields. Continental tholeiites, such as the Columbia River basalts, are high in P₂O₅ relative to MnO and TiO₂, and overlap portions of all five oceanic fields.MnO is depleted relative to TiO₂ in mid-ocean ridge analyses and may be controlled by early fractionation of olivine and/or clinopyroxene under conditions of lowf_O2. In island arc rocks, MnO is enriched relative to TiO₂ due to early crystallization of titanomagnetite in a high-f_O2 environment. Primitive mid-ocean ridge and arc tholeiites have similar MnO/TiO₂/P₂O₅ ratios which indicate a grossly similar parent magma. Increasingly differentiated basaltic rocks are more easily classified by the diagram. High relative abundances of TiO₂ and P₂O₅ in ocean island rocks are consistent with their derivation from a separate source.Despite the purported high mobility of MnO, the MnO/TiO₂/P₂O₅ discriminant diagram may be applied to unspilitized and moderately spilitized zeolite to greenschist facies greenstones with good agreement between the environment determined by MnO/TiO₂/P₂O₅ and by other means such as trace elements, REE, or field relations.     

Depositional model of Early Permian reef-island ocean in Eastern Kunlun

Many fusulinid fossils have been found in thin- to middle-bedded limestones which are distributed between the Early Permian limestone hills and formerly considered as Early Triassic. The fusulinid fossils, identified asNeoshwagerina sp.,Verbeekina sp. andSchwagerina sp., can also be found in massive limestone hills. At the same time, Early Permian radiolarian chert of deep basin facies was discovered in Animaqing. All the above show that the massive limestone hills, thin- to middle-bedded limestones and radiolarian chert belong to syndeposits in Early Permian ocean. The sediments in the study area can roughly be divided into three types: shallow facies, basin facies and transitional facies. The carbonate buildup can be subdivided into massive bioclastic limestone and reef framestone. Basin facies contains thin- or middle-bedded limestone, abyssal red mudstone or ooze, blue-green mudstone and radiolarian chert. Transitional facies includes reef talus and platformal skirt facies. The Early Permian ocean in Eastern Kunlun is recognized as a kind of reef-island ocean environment according to distribution and composition of different facies. The reef-island ocean in Eastern Kunlun is characterized by reef islands (or carbonate buildups) alternating with basins, complicated sea-floor topography, sharp facial change and well-developed reefs.     

Early Oligocene alkaline lamprophyric dykes from the Jandaq area (Isfahan Province,Central Iran): Evidence of Central–East Iranian microcontinent confining oceanic crust subduction

The Jandaq lamprophyres occur as eight mostly parallel dykes, which cross‐cut Eocene volcanic and sedimentary rocks of the Pis‐Kuh Formation in dominant north to south direction. These lamprophyres are mainly composed of kaersutite, clinopyroxene, olivine, feldspar, ilmenite, and spinel as primary minerals. The rocks studied here are enriched in alkalis, TiO₂, large ion lithophile elements, and light rare‐earth elements (LREE), with SiO₂ content between 41.7 and 46.2 wt%, and are classified as camptonite and alkaline lamprophyre according to the mineralogical and chemical characteristics. These rocks exhibit positive Eu anomalies (Eu/Eu* = 1.08–1.39) and are characterized by strong enrichment in LREE relative to heavy REEs, and also by varied Zr/Hf ratios. The geochemical features of the rocks suggest that the lamprophyre magmas were derived from low‐degree melting of an amphibole garnet lherzolite that experienced strong metasomatism by carbonate‐rich fluids in response to dehydration melting from the subducted slab. The Jandaq lamprophyric magmatism has been attributed to the former subduction of the Central–East Iranian microcontinent confining oceanic crust from the Triassic to Eocene, and decompression melting induced by the extensional basin of the Jandaq area in the early Oligocene.     

Petrology,geochemistry, and age of the Spurr volcanic complex,eastern Aleutian arc

The Spurr volcanic complex (SVC) is a calc-alkaline, medium-K, sequence of andesites erupted over the last 250000 years by the eastern-most currently active volcanic center in the Aleutian arc. The ancestral Mt. Spurr was built mostly of andesites of uniform composition (58%–60% SiO₂), although andesite production was episodically interrupted by the introduction of new batches of more mafic magma. Near the end of the Pleistocene the ancestral Mt. Spurr underwent avalanche caldera formation, resulting in the production of a volcanic debris avalanche with overlying ashflows. Immediately afterward, a large dome (the present Mt. Spurr) formed in the caldera. Both the ash flows and dome are made of acid andesite more silicic (60%–63% SiO₂) than any analyzed lavas from the ancestral Mt. Spurr, yet contain olivine and amphibole xenocrysts derived from more mafic magma. The mafic magma (53%–57% SiO₂) erupted during and after dome emplacement from a separate vent only 3 km away. Hybrid block-and-ash flows and lavas were also produced. The vents for the silicic and mafic lavas are in the center and in the breach of the 5-by-6-km horseshoe-shaped caldera, respectively, and are less than 4 km apart. Late Holocene eruptive activity is restricted to Crater Peak, and magmas continue to be relatively mafic. SVC lavas are plag ±ol+cpx±opx+mt bearing. All postcaldera units contain small amounts of high-Al₂O₃, high-alkali amphibole, and proto-Crater Peak and Crater Peak lavas contain abundant pyroxenite and anorthosite clots presumably derived from an immediately preexisting magma chamber. Ranges of mineral chemistries within individual samples are often nearly as large as ranges of mineral chemistries throughout the SVC suite, suggesting that magma mixing is common. Elevated Sr, Pb, and O isotope ratios and trace-element systematics incompatible with fractional crystallization suggest that a significant amount of continental crust from the upper plate has been assimilated by SVC magmas during their evolution.