Geochemistry Of Volcanic Rocks Research Articles

Geochronology, geochemistry, and tectonic setting of Ordovician metavolcanic rocks in the Liberty–Orrington belt, Maine: implications for the evolution of peri-Gondwanan arcs in the northern Appalachians

The Yarmouth Island Formation of the East Harpswell Group (EHG) is an assemblage of metamorphosed volcanic and sedimentary rocks exposed in Casco Bay, Maine. Although previously interpreted to have been deposited ca. 445 Ma, two new U–Pb zircon ages from metavolcanic rocks in the unit indicate prolonged magmatism between ca. 469–474 Ma and correlation with adjacent rocks of the more regionally extensive Casco Bay Group (CBG) in the Liberty–Orrington belt. New detrital zircon results from metasedimentary rocks in the EHG support this correlation; the age spectra indicate a peri-Gondwanan sedimentary source, consistent with earlier published results from the CBG. Detailed geochemical studies of the metavolcanic rocks in the Yarmouth Island Formation indicate a wide range of sub-alkaline compositions (SiO2 = 47.9–77.5 wt.%), flat chondrite-normalized rare earth element distributions, and a prominent negative Nb anomaly in normalized spider plots. Tectonic discrimination plots suggest eruption in a volcanic arc setting with continental crustal influence. This tectonic setting, when combined with the co-existence of marine sedimentary rocks, suggests a transitional setting between that of subduction and subsidence in a marine setting, perhaps due to the onset of back-arc rifting. Similarities in age, lithologic character, sediment provenance, and volcanic rock geochemistry indicate a strong correlation between these rocks in the Liberty–Orrington belt and those along strike in the southern portion of the Miramichi belt of eastern Maine and southern New Brunswick. These findings confirm the preservation of >500 km long, ca. 470 Ma, arc/back-arc terrane within the Ganderia superterrane in the northern Appalachians.

Petrography, mineralogy and geochemistry of volcanic rocks in Gorazi area, southwest Khaf, Razavi Khorasan Province

Petrography, mineralogy and geochemistry of volcanic rocks in Gorazi area, southwest Khaf, Razavi Khorasan Province

Geochemistry of volcanic rocks and dykes from the Remeshk-Mokhtarabad and Fannuj-Maskutan Ophiolites (Makran Accretionary Prism, SE Iran): New constraints for magma generation in the Middle East neo-Tethys

The Remeshk-Mokhtarabad and Fannuj-Maskutan ophiolites represent two major ophiolitic units in the North Makran Domain (Makran Accretionary Prism). Volcanic rocks and dykes of these ophiolites mainly consist of basalts and rare basaltic andesites, andesites and dacites. No chemical distinction can be seen in basalts from these two ophiolitic units, or between volcanic rocks and dykes. Basaltic rocks show a broad MORB-type nature but variable chemical composition (e.g., SiO2= 42.64–52.63 wt%; TiO2= 0.98–2.43 wt%; Mg# = 71–50). They show both N-MORB (Type 1) and E-MORB (Type 2) compositions (MORB: mid-ocean ridge basalt; N: normal; E: enriched). Type 1 rocks are very rare in both ophiolitic units, whereas Type 2 rocks are predominant. Type 1 rocks show low Th (0.10–0.16 ppm), Nb (1.86–2.82 ppm), Ta (0.09–0.17 ppm) abundance and low (La/Yb)N (0.50–0.75), (La/Sm)N (0.48–0.72) ratios. Compared to N-MORBs, Type 2 basalts show slight enrichment in Th (0.42–1.60 ppm), Nb (6.09–14.6 ppm), and Ta (0.227–0.792 ppm), as well as (La/Yb)N and (La/Sm)N ratios >1 like those observed in typical E-MORB. Trace element petrogenetic models indicate that primitive basalts derived from partial melting of a heterogeneous sub-oceanic mantle variably metasomatized by plume-type (OIB-) components. Type 1 basalts derived from partial melting of mantle regions with no enrichment in OIB-type components, whereas Type 2 basalts derived from partial melting of DMM sources variably enriched by OIB-components. These rocks formed in an oceanic basin that was strongly affected by mantle plume activity and different extents of plume-ridge interaction.

Late Paleozoic Tectonomagmatic Evolution of the Eastern Tianshan, Northwest China: Insights from Geochronology and Geochemistry of Volcanic Rocks from the Dananhu–Lop Nur Area

Late Paleozoic Tectonomagmatic Evolution of the Eastern Tianshan, Northwest China: Insights from Geochronology and Geochemistry of Volcanic Rocks from the Dananhu–Lop Nur Area

An Early Cretaceous arc–back-arc system in western Yunnan, SW China: Constraints from U Pb zircon ages and geochemistry of volcanic rocks in the western Baoshan block

Whether the voluminous Early Cretaceous magmatism in the Tengchong and Baoshan blocks, western Yunnan, China, was related to subduction of the Meso-Tethys or the Neo-Tethys is controversial. Here, we report SHRIMP and LA-ICP-MS UPb zircon ages, bulk-rock major and trace elements, and SrNd isotope data from volcanic rocks in the Lameng area of the Baoshan block, SW China, investigate their genesis, and discuss the evolution of the Tengchong and Baoshan blocks. These rocks yield UPb zircon ages of 120.3 ± 1.7 and 120.8 ± 1.4 Ma and constitute a bimodal volcanic suite (group 1 and group 2). Group 1 samples show alkaline basaltic signatures (SiO2 = 46.2–53.1 wt.%), enrichments in Zr, Hf, Ti, and light rare earth elements, and slight depletions in Nb, Ta, and heavy rare earth elements. They have εNd(t) = −1.0–+3.7 and initial 87Sr/86Sr = 0.70503–0.70709. Group 2 samples display higher silica contents (SiO2 = 61.7–65.0 wt.%) and evolved εNd(t) (−4.3 to −3.6) and initial 87Sr/86Sr (0.70653–0.70678) values. These features suggest that the group 1 melts were derived from an asthenospheric mantle source mixed with subcontinental lithospheric mantle components and that the group 2 samples were produced by partial melting of the lower crust in a continental back-arc setting. From an affinity with widespread coeval magmatic rocks in the Tengchong and Lhasa blocks, we suggest that the Bangong-Nujiang Ocean between the Tengchong and Baoshan blocks had closed by 120 Ma and that the Early Cretaceous magmatism in the Tengchong and Baoshan blocks formed in an arc–back-arc system associated with Neo-Tethyan slab rollback.

Constraints on the rollback of the Neo-Tethyan oceanic plate: Geochronology and geochemistry of volcanic rocks from the Dianzhong Formation, western Gangdese belt (Tibetan Plateau)

In this contribution, we report new U-Pb SHRIMP zircon ages and geochemical data for volcanic rocks from the Dianzhong Formation. The unit represents the lower part of the Linzizong volcanic succession located in the Shiquanhe area, western part of the Gangdese belt. Zircon U-Pb dating of three trachytes yields ²⁰⁶Pb/²³⁸U crystallization ages of 71.5±0.6 Ma, 70.8±1.0 Ma and 68.9±1.0 Ma. Whole rock major and trace element analyses indicate a main trend of calc-alkalic to high-potassic calc-alkalic, as well as an enrichment in large ion lithophile elements (LILEs), depletion in high field strength elements (HFSEs), with negative Nb, Ta and Ti anomalies. In addition, these volcanic rocks are enriched in LREE and depleted in HREE without a clear Eu anomaly. These geochemical features are similar to those of active continental margin volcanic rocks. In combination with previously published zircon U-Pb ages and geochemical data of the Dianzhong Formation, we suggest that the Linzizong volcanism started as early as the late Cretaceous (∼71 Ma) and the volcanic rocks of the Dianzhong Formation in the Shiquanhe area are the product of rollback of the Neo-Tethyan oceanic plate.

Petrographic Characteristics and Geochemistry of Volcanic Rocks in the Kyaukmyet Prospect, Monywa District, Central Myanmar

The Kyaukmyet prospect lies approximately 5 km ENE of the highsulfidation Kyisintaung copper-gold deposit, Monywa district, central Myanmar. Geologically, the research area is remarked by magmatic extrusion that occurred during the Late Oligocene to Middle Miocene of Magyigon Formation which led to the outcrops of volcanic rocks. Study detailed on petrographical and geochemical of the Kyaukmyet volcanic rocks has not been performed before the present work. The principal aim of this paper is to document the petrographical and geochemical characteristics of volcanic suite rocks exposed in the Kyaukmyet prospect. The results of this data have provided insight into the origin of the rocks and petrogenetic processes during evolution. Petrographically, all the studied volcanic rocks in the research area show that trachytic and porphyritic textures with phenocrysts of quartz, plagioclase, and K-feldspar which are embedded in a fine to medium grained groundmass. The accessory minerals of this rock consist of biotite, chlorite and opaque mineral.Geochemically, these volcanic rocks having calc-alkaline nature and classified as volcanic field (rhyolite) as well as volcanic arc setting. Based on the chondrite normalized spider diagram, LREE has enriched to HREE in this area which indicated negative Eu anomaly and subduction tectonic setting.

Paleoproterozoic (2.51–2.40 Ga) Igneous Provinces of the Northeastern Fennoscandia: Geochemistry of Volcanic Rocks and Correlation with Intrusive Complexes

The results of geochemical studies of Paleoproterozoic volcanic rocks in the structure of the Polmak–Pechenga–Imandra–Varzuga belt of the northeastern part of the Fennoscandian Shield are presented. Taking into account new geochronological data, the volcanic series are correlated with subvolcanic and intrusive rocks of the Murmansk and Kola–Norwegian terranes. The comparative analysis of sections of volcanic rocks of the Imandra–Varzuga and Pechenga structures indicate asynchronous evolution of these fragments of the belt in a range of 2.51–2.40 Ga and a significant time gap (probably, >300 m.y.) in evolution of both the Imandra–Varzuga and Pechenga structures. The geochemical evidence is provided for the cyclic plume-lithospheric interaction during magmatism of the Sumian–Sariolian period, which shows that basic melts mostly generated from facies of spinel lherzolites exhibit features of significant crustal contamination as a result of their interaction with rocks of the Archean basement.

Cryptic alkaline magmatism in the oceanic Caribbean arc (Camagüey area, Cuba)

Mid and Late Cretaceous arc-related basaltic rocks in the Camagüey area, East Central Cuba, record an intriguing seemingly random enrichment in alkalis that suggests alkaline affinity, representing an oddity in the Greater Antilles. In this study, the petrology and geochemistry of volcanic rocks from the Camujiro, Piragua and La Mulata Formations have been investigated. The rocks bear a diverse mineral assemblage including feldspars (plagioclase and potassium feldspar), clinopyroxene, phlogopite and amphibole with both equilibrium and disequilibrium textures. Clinopyroxene porphyritic crystals show conspicuous oscillatory and convolute zoning and reveal events of mafic recharge and crystallization in equilibrium with alkaline and calc-alkaline basaltic magmas and their mixtures. Composition of fluorophlogopite xenocrystals is consistent with crystallization from mafic alkaline liquids.Unlike whole-rock major element composition, immobile trace element abundances suggest a systematic calc-alkaline affinity. Chondrite-normalized rare-earth element (REE) patterns are similar to those of calc-alkaline and high-K (and Th) calc-alkaline rocks from the Caribbean Antilles. Geochemical modelling indicates that the parental basic magmas formed by a 5–10% hydrous partial melting of spinel-facies depleted mantle fluxed by fluids evolved from subducted oceanic crust and pelagic sediments, as well as fractional crystallization. Random enrichment in alkalis in studied samples is attributed to the entrainment during ascent of material derived from mafic alkaline melts injected in the upper mantle or the root of the island-arc.Our new data establish a mature stage in the evolution of the Caribbean island-arc by Cenomanian times capable of sourcing alkaline melts and the existence of a thickened island arc crust that favored fractional crystallization and magma chamber recharge processes unlike in previous stages of the island-arc construction.

Early Paleozoic tectonic evolution and magmatism in the Eastern Tianshan, NW China: Evidence from geochronology and geochemistry of volcanic rocks

The Eastern Tianshan provides significant insights in relation to unraveling the history of Early Paleozoic magmatism, ocean-continent transition, and tectonic evolution of the southern Central Asian Orogenic Belt. This investigation represents a detailed geochronological and geochemical study of the Early Paleozoic volcanic rocks from the Harlik and Dananhu terranes in the Eastern Tianshan. UPb zircon ages indicate periods of volcanism in Harlik Early Silurian (441–434 Ma) and Dananhu Late Ordovician (448–445 Ma). The Harlik Early Silurian volcanic rocks unconformably overlie Ordovician sediments, and consist of alkaline (434 Ma) and tholeiitic (440 Ma) basalts. The alkaline basalts originated from low degree partial melting of previously subduction-modified lithosphere and subsequent interaction with upwelling asthenosphere. The tholeiitic basaltic magmas were derived from partial melting of a spinel facies lithospheric mantle source metasomatized by early slab-derived fluids, which then underwent crystal fractionation of clinopyroxene and olivine. In contrast, the Dananhu Late Ordovician high-Mg andesites were generated by partial melting of a hydrous, depleted mantle wedge and these melts experienced a low degree of crystal fractionation. Combined with results from previous geological studies, it is proposed that the Harlik and Dananhu terranes had different volcanism and tectonic histories. The Harlik tholeiitic basalts were generated in a post-collision setting, which may be associated with the closure of the Armantai Ocean. Contrarily, the alkaline basalts were formed in an intraplate setting during the initial stages of intracontinental extension that gave rise to the Karamaili Ocean. The Dananhu Late Ordovician andesites represent the initial phase of arc-related volcanism due to the subduction of the North Tianshan Ocean, where the Dananhu arc may have temporally evolved from an Andean- to oceanic island-type arc in Early Paleozoic.

Mafic Magmatism of Northeastern Fennoscandia (2.06–1.86 Ga): Geochemistry of Volcanic Rocks and Correlation with Dike Complexes

The comprehensive geochemical and isotopic-geochronological study of Early Proterozoic volcanic rocks in structure of the Polmak–Pechenga–Imandra–Varzuga belt and dikes and sills of the Murmansk and Kola–Norwegian terranes is conducted. Abundant swarms of mafic dikes (2.06–1.86 Ga) are established in the northwestern frame of the belt, including swarms of metadolerites (2060 ± 6 Ma), ferropicrites and gabbronorites (1983 ± 5 Ma), and poikilophitic dolerites (1860 ± 4 Ma). The comparison of volcanic rocks of the Pechenga and Imandra–Varzuga structures shows asynchronous change in volcanism style, with a significant time lapse. The geochemical features of volcanic rocks of the Tominga Formation are typical of those of continental magmatism and can hardly be correlated with those of the Pilguyarvi Formation. According to isotopic-geochronological data, depleted mantle melts in the Pechenga and Imandra–Varzuga zones intruded at 2010–1970 and 1970–1980 Ma, respectively. The analysis of the conditions of formation of volcanic series shows that Neoarchean lithospheric mantle, which produced melts with low Zr/Nb ratios, was a source for primary melts of the Kuetsjarvi Formation of the Pechenga structure and their homologs of the Imandra–Varzuga structure. In contrast, the volcanic rocks of the Kolasjoki Formation, which were weakly contaminated with crustal material, and the related Ilmozero Formation, as well as the metadolerite dikes of the Kirkenes region, were sourced mostly from asthenosphere with separation of melt above the garnet stability depth. The formation of the volcanic rocks of the Pilguyarvi Formation is related, judging from the geochemical data, to two asthenospheric sources different in depth, which produced tholeiitic and ferropicritic melts.

Geochemistry of volcanic rocks from Oldoinyo Lengai, Tanzania: Implications for mantle source lithology

Oldoinyo Lengai is the only active carbonatite volcano within the East African Rift Valley in northern Tanzania. The volcano is dominated by peralkaline silicate rocks with natrocarbonatites. This study presents new mineralogical and geochemical data, including Sr–Nd–Pb–Hf–Mg isotopic compositions, for volcanic rocks at Oldoinyo Lengai and lavas from the nearby Gregory Rift Valley. The samples analyzed in this study include olivine melilitite, melanephelinite, wollastonite nephelinite, and phonolite. The olivine melilitites and melanephelinites have highly fractionated rare-earth-element (REE) patterns with (La/Yb)N values of 26.4–64.9, suggesting that they formed from magmas generated by low-degree (up to ∼7%) of partial melting within the garnet stability field. The wollastonite nephelinites have much higher (La/Sm)N values but lower (Sm/Yb)N values relative to typical oceanic island basalts (OIB), with flat heavy REE patterns [(La/Yb)N = ∼22]. The phonolites have elevated REE abundances but with patterns intermediate between the other two sample groups [(La/Yb)N = ∼41]. All samples have primitive-mantle-normalized incompatible element patterns that are characterized by negative K and Rb anomalies but no significant Eu anomalies. They also have elevated Yb contents relative to the compositions of modeled garnet peridotite-derived melts, suggesting that they were derived from a sublithospheric source containing enriched HIMU-like recycled oceanic crustal material. However, the wollastonite nephelinites have significantly positive Ba, U, Sr, and Pb anomalies similar to those found within the Oldoinyo Lengai natrocarbonatites. The wollastonite nephelinites might have been sourced from a region of sub-continental lithospheric mantle (SCLM) that was previously metasomatized by interaction with carbonatite melts. The phonolites in the study area have also weakly positive Pb and Sr anomalies indicative of some interaction with the SCLM. All samples have δ26Mg values (−0.39‰ ± 0.07‰) lighter than the composition of normal mantle material (−0.25‰ ± 0.04‰). In addition, a negative correlation between δ26Mg values and MgO concentrations suggests derivation from a source region containing recycled carbonate. The samples from the study area define a mixing array between HIMU- and EM1-type OIB in Sr–Nd and Pb–Pb isotopic correlation diagrams, and have pronounced Nd–Hf isotopic decoupling, plotting below the mantle regression line in Nd–Hf isotopic space. The negative deviation from the Nd–Hf isotopic mantle array and the presence of an EM1-type mantle component in the Sr–Nd isotopic compositions of the Oldoinyo Lengai volcanic rocks can be generated by recycling of E-MORB-type oceanic crustal material with an age of 1.5–1.0 Ga.

Zircon U–Pb dating and whole‐rock geochemistry of volcanic rocks in eastern Heilongjiang Province, NE China: Implications for the tectonic evolution of the Mudanjiang and Paleo‐Pacific oceans from the Jurassic to Cretaceous

The eastern Heilongjiang Province, NE China, is located in the eastern CAOB (Central Asian Orogenic Belt). Previous studies suggest that subduction of the Mudanjiang and Paleo‐Pacific oceans had been initiated in this area during the early Mesozoic. However, research on the Mesozoic volcanic rocks is insufficient, limiting our understanding of the relationship between magmatism and subduction process. To solve the problem, seven volcanic rocks in the Lesser Xing'an‐Zhangguangcai Range and the Jiamusi Block were collected for zircon U–Pb dating (SHRIMP) and whole‐rock geochemical analysis. Most samples from the Lesser Xing'an‐Zhangguangcai Range are Jurassic in age (189–178 Ma), including andesite, trachyandesite, trachydacite, and rhyolite. The andesitic to trachytic rocks were generated by magma mixing at an active continental margin, whereas the rhyolite is highly fractionated I‐type. All the samples from the Jiamusi Block and one basaltic andesite from the Lesser Xing'an Range are late Early Cretaceous in age (ca. 100 Ma) and were formed in a back‐arc extensional setting (roll‐back). Fractional crystallization was important in the generation of most Cretaceous rocks (basalt to trachydacite). However, magma mixing or crustal contamination was required to explain some andesites from the Jiamusi Block showing extremely low Cr, Ni, and Mg#. When integrated with the coeval blueschist‐facies metamorphism, the Early Jurassic volcanism indicates an active continental margin in the Lesser Xing'an‐Zhangguangcai Range due to the westward subduction of the Mudanjiang Ocean. After the closure of the Mudanjiang Ocean, two NE/NNE trending sinistral strike‐slip faults in NE China were triggered, accompanied by the slab roll‐back of the Paleo‐Pacific oceanic plate. The Jurassic continental arc and accretionary complexes in NE China were cut through by the two strike‐slip faults and bended to form small oroclines. At ca. 100 Ma, numerous accretionary complexes were accreted along the eastern margin of the CAOB, further forcing the retreat of trench and slab roll‐back. The late Early Cretaceous intra‐continental volcanism was generated in the Lesser Xing'an‐Zhangguangcai Range and Jiamusi Block during this roll‐back process.

Geology, Petrology and Geochemistry of Volcanic Rocks Around Arba Minch, Southern Ethiopia

The study area is subpart of the southern main Ethiopian Rift and geographically bounded between UTM (Universal Transverse Mercator) coordinates of 662000m-669000m latitude and 336000m-341000m longitude. The main aim of this study is to investigate the geological, petrographic and geochemical properties of volcanic rocks around Arba Minch, southern Ethiopia. Both field and laboratory analytical techniques are employed to characterize minerals and the rock types. Eight thin section samples are prepared to study petrographic properties of minerals and five best representative samples are analyzed for whole rock chemistry using ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). Petrographically, samples from the porphyritic basalt show phenocrysts of olivine and plagioclase within the fine-grained groundmass. There are rounded to subrounded holes or vesicles that appear white under plane polarized light observations and dark under cross polarized light observations which further confirms vesicular nature of the basalt. Thin section sample from rhyolite outcrop shows phenocrysts of K-feldspar and quartz within tiny fine-grained groundmass of feldspar minerals. Aphanitic basalt, Porphyritic basalt, vesicular basalt, rhyolite and dolerite are the rock units found in the study area. The geochemical results of the analyzed samples indicate silica ranges of (48.5-60.1)%. Based geochemical classifications made, rock units are classified into rhyolite, basalt, hawaiite and mugherite rock types. Trace element analytical results show all samples lie on the within plate tectonic setting. The rare earth and trace element ratios and spider diagrams confirmed that most samples are derived from magmas of asthenospheric source with little crustal contamination.

The Thickness of the Mantle Lithosphere and Collision-Related Volcanism in the Lesser Caucasus

The Lesser Caucasus mountains sit on a transition within the Arabia–Eurasia collision zone between very thin lithosphere (<100 km) to the west, under Eastern Anatolia, and a very thick lithospheric root (up to 200 km) in the east, under western Iran. A transect of volcanic highlands running from NW to SE in the Lesser Caucasus allows us to look at the effects of lithosphere thickness variations on the geochemistry of volcanic rocks in this continental collision zone. Volcanic rocks from across the region show a wide compositional range from basanites to rhyolites, and have arc-like geochemical characteristics, typified by ubiquitous negative Nb–Ta anomalies. Magmatic rocks from the SE, where the lithosphere is thought to be thicker, are more enriched in incompatible trace elements, especially the light rare earth elements, Sr and P. They also have more radiogenic ⁸⁷Sr/⁸⁶Sr, and less radiogenic ¹⁴³Nd/¹⁴⁴Nd. Across the region, there is no correlation between SiO₂ content and Sr–Nd isotope ratios, revealing a lack of crustal contamination. Instead, ‘spiky’ mid-ocean ridge basalt normalized trace element patterns are the result of derivation from a subduction-modified mantle source, which probably inherited its subduction component from subduction of the Tethys Ocean prior to the onset of continent–continent collision in the late Miocene. In addition to the more isotopically enriched mantle source, modelling of non-modal batch melting suggests lower degrees of melting and the involvement of garnet as a residual phase in the SE. Melt thermobarometry calculations based on bulk-rock major elements confirm that melting in the SE must occur at greater depths in the mantle. Temperatures of melting below 1200°C, along with the subduction-modified source, suggest that melting occurred within the lithosphere. It is proposed that in the northern Lesser Caucasus this melting occurs close to the base of the very thin lithosphere (at a depth of ∼45 km) as a result of small-scale delamination. A striking similarity between the conditions of melting in NW Iran and the southern Lesser Caucasus (two regions between which the difference in lithosphere thickness is ∼100 km) suggests a common mechanism of melt generation in the mid-lithosphere (∼75 km). The southern Lesser Caucasus magmas result from mixing between partial melts of deep lithosphere (∼120 km in the south) and mid-lithosphere sources to give a composition intermediate between magmas from the northern Lesser Caucasus and NW Iran. The mid-lithosphere magma source has a distinct composition compared with the base of the lithosphere, which is argued to be the result of the increased retention of metasomatic components in phases such as apatite and amphibole, which are stabilized by lower temperatures prior to magma generation.

Geochemical Discrimination and Characteristics of Magmatic Tectonic Settings: A Machine‐Learning‐Based Approach

AbstractGeochemically discriminating between magmatism in different tectonic settings remains a fundamental part of understanding the processes of magma generation within the Earth's mantle. Here we present an approach where machine learning (ML) methods are used for quantitative tectonic discrimination and feature selection using global geochemical data sets containing data for volcanic rocks generated in eight different tectonic settings. This study uses support vector machine, random forest, and sparse multinomial regression (SMR) approaches. All these ML methods with data for 24 elements and five isotopic ratios allowed the successful geochemical discrimination between igneous rocks formed in eight different tectonic settings with a discriminant ratio better than 83% for all settings barring oceanic plateaus and back‐arc basins. SMR is a particularly powerful and interpretable ML method because it quantitatively identifies geochemical signatures that characterize the tectonic settings of interest and the characteristics of each sample as a probability of the membership of the sample for each setting. We also present the most representative basalt composition for each tectonic setting. The new data provide reference points for future geochemical discussions. Our results indicate that at least 17 elements and isotopic ratios are required to characterize each tectonic setting, suggesting that geochemical tectonic discrimination cannot be achieved using only a small number of elemental compositions and/or isotopic ratios. The results show that volcanic rocks formed in different tectonic settings have unique geochemical signatures, indicating that both volcanic rock geochemistry and magma generation processes are closely connected to the tectonic setting.

Geochronology and geochemistry of volcanic rocks from the Jingtan Formation in the eastern Jiangnan orogen, South China: Constraints on petrogenesis and tectonic implications

An integrated study of zircon U-Pb geochronology and geochemistry, together with Nd-Hf isotopes, have been carried out on the rhyodacite and rhyolitic tuff of the Jingtan Formation in the eastern part of the Jiangnan orogen. SIMS zircon U-Pb dating of two samples yielded weighted mean 206Pb/238U ages of 784±6Ma and 788±6Ma, respectively. Geochemically, they are peraluminous (A/CNK mostly around 1.2) and are characterized by enrichments in Rb, Th, REEs and HFSEs (e.g. Zr and Y) but depletions in Ba, Sr, P, Eu and Ti. The volcanic rocks show a clear A-type granite geochemical signature with high total alkalis (K2O+Na2O=5.3–7.64wt%), FeOt/MgO ratios and low CaO, MgO and TiO2 contents. They have negative whole-rock εNd(t) (−4.2 to −1) and positive zircon εHf(t) (+1.26 to +11.6) values, illustrating decoupled Nd-Hf isotopes which may be genetically related to their petrogenesis process. The positive εHf(t) values and juvenile THfDM1 (0.89–1.3Ga) of zircons indicate that the volcanic rocks may have been derived from the partial melting of the Neoproterozoic to late Mesoproterozoic crustal materials. Combined with the occurrence of significant volumes of contemporary bimodal volcanic rocks in eastern section of the Jiangnan orogen, it is inferred that the Jingtan felsic volcanic rocks formed during post-collisional extension shortly after the final amalgamation of Yangtze and Cathaysia blocks.

The formation mechanism of accretionary wedge at Karamay in West Junggar, NW China

Accretionary wedge is the typical product of subduction-zone processes at shallow depths. Determining the location, composition and mechanism of accretionary wedge has important implications for understanding the tectonism of plate subduction. The Central Asian Orogenic Belt (CAOB) is one of the world’s largest accretionary orogenic belts, and records the bulk evolution of Paleo-Asian Ocean from opening to closure, with multi-stages and multi-types of crust-mantle interaction in the Paleozoic. West Junggar (western part of Junggar Basin), located in the core area of CAOB, is characterized by a multiple intra-oceanic subduction system during the Paleozoic. In its eastern part crop out Devonian-Carboniferous marine sedimentary rocks, Darbut and Karamay ophiolitic melanges, alkali oceanic island basalts, island arc volcanic rocks and thrusted nappe structure. Such lithotectonic associations indicate the occurrence of accretionary wedge at Karamay. In order to decipher its formation mechanism, this paper presents a synthesis of petrography, structural geology and geochemistry of volcanic rocks. In combination with oceanic subduction channel processes, it is suggested that the accretionary wedge is a composite melange with multiple stages of formation. The application of oceanic subduction channel model to the Karamay accretionary wedge provides new insights into the accretion and orogenesis of CAOB.

Space–time relationships between volcanic associations of different alkalinities: The Belogolovskii Massif, Sredinnyi Range, Kamchatka. Part II. Geochemistry of volcanic rocks and magma sources

Data are presented relating to volcanic series in the Belogolovskii Massif, Sredinnyi Range, Kamchatka. We discuss new geochronologic data, the distributions of rare elements and platinum elements in the rocks, and list the isotope characteristics of volcanic series with normal and moderate alkalinities. We show that the Late Pliocene to Early Pleistocene rocks that belong to the moderate alkaline series of the Belogolovskii volcanic massif are different from rocks in the normally alkaline series of the Late Miocene to Middle Pliocene volcanogenic basement in having higher concentrations of the HFSE and LILE components. We propose a model for the generation of moderate alkaline magmas involving a heterogeneous depleted and a heterogeneous enriched source of material. According to the isotope data, one of these sources may be the subducted oceanic lithosphere of the Pacific and the Commander-Islands type, while the other source was recycled material of the Indian MORB type.

Geochronology and geochemistry of volcanic rocks in the Arbasay Formation, Xinjiang Province (Northwest China): implications for the tectonic evolution of the North Tianshan

ABSTRACTThe extensively exposed late Carboniferous volcanic and volcaniclastic successions along the northern margin of the North Tianshan are called the Arbasay Formation. We present field-based mapping, petrography, zircon cathodoluminescence (CL) images, and U–Pb dates, as well as whole-rock geochemical data for these rocks, in order to constrain their formation age and petrogenesis, and understand the geodynamic setting. Conspicuously, the Arbasay Formation shows typical basalt–andesite–dacite–rhyolite volcanic series, and is dominated by andesites with a small amount of basalts and rhyolites based on the geological profile. U–Pb isotopic dating using the LA-ICP-MS method on zircons reveals that the volcanic rocks in the Arbasay Formation formed at 308–305 Ma, that is, late Carboniferous, rather than early Permian as previously proposed. Geochemically, the volcanic rocks mainly belong to the calc-alkaline series and have arc-like geochemical compositions. They are enriched in LREEs ((La/Yb)N = 2.9–7.5) and LILEs (K, Rb, Ba) and depleted in HFSEs (Nb, Ta, Ti). In the tectonic discrimination diagrams, the basalts mainly fall into the area of continental arc. Given our U–Pb dating results, geochemical characteristics, and the regional geological framework, we propose that the late Carboniferous volcanic rocks originated from the arc-related setting, not the intracontinental rift-related setting. They are possibly the major constituents of a continental arc that is formed with the southward subduction of the North Tianshan Oceanic lithosphere.