Continental Margin Magmatic Arc Research Articles

Lithology, source areas and formation settings of upper triassic deposits of South-Western Primorye

The article considers the material composition of sandy rocks from the Upper Triassic deposits of the Mongugai formation of South-Western Primorye and presents the results of its paleogeodynamic interpretation. It has been established that, according to their mineralogical and geochemical parameters, sandstones are petrogenic, correspond to graywackes and, only partially, to lithite arenites, are characterized by a significant degree of geochemical maturity of the clastic material, and their formation occurred due to the erosion of significantly weathered parent rocks of the source areas. Generalization and paleogeodynamic interpretation of the data obtained indicate that in the Late Triassic in the southwestern part of Primorye, sedimentation occurred in basins associated with the environment of the active continental margin and, probably, complicated by strike-slip dislocations along transform faults. The dominant source of clastic matter was continental land: cratons and projections of crystalline basement framed by rift zones or along transform faults. Acidic granite-metamorphic complexes were eroded with the participation of sedimentary rocks enriched with ancient components. A secondary source was a deeply dissected continental margin magmatic arc, which supplied the basin with an additional amount of sialic material, as well as a small amount of the basic-medium volcanoclastic. U-Pb geochronological studies of detrital zircons from sandy rocks of the formation made it possible to establish the age and possible location of magmatic complexes, due to the destruction of which the deposits were formed.

Neoproterozoic transition from subduction to breakup along the margin of the Rodinia supercontinent: Sedimentary evidence from the northwestern Tarim Block, Northwest China

The Tarim Block, northwest China, was positioned on the periphery of Rodinia during the Neoproterozoic Era, and preserves a geological archive chronicling the assembly and breakup of the supercontinent. Due to its antiquity and limited exposure, sedimentary records along the margin of the Rodinia supercontinent remain sparsely documented. This study focuses on the Xifangshan and Dongqiaoenbrak formations, the oldest sedimentary units in the northwestern Tarim Block, which comprise a Neoproterozoic sedimentary record at the supercontinent margin during the transition from subduction to breakup. A comprehensive analysis employing sedimentology, petrology, elemental geochemistry, and UPb detrital zircon chronology was undertaken, to elucidate paleoenvironment, parent rocks, and tectonic-sedimentary evolution. Findings show that the Neoproterozoic witnessed the subduction of an oceanic plate beneath the Tarim Block, resulting in the formation of a continental margin magmatic arc. The Xifangshan Formation, deposited during the late phase of this arc (with the earliest sedimentary age recorded at 770 Ma), represents a submarine fan dominated by turbidity currents. Around 760 Ma, rapid crustal uplift occurred, and the tectonic setting gradually shifted from subduction to rift, giving rise to the coastal alluvial fan sedimentary system observed in the Dongqiaoenbrak Formation. Both the Xifangshan and Dongqiaoenbrak Formations record deposition in an oxic to suboxic marine environment with moderate salinity, influenced by a semiarid to semihumid palaeoclimate. The continental margin arc served as the primary source for sedimentation in both formations, and a few magmatic products linked to extension (syenite, monzonite) were also cannibalized into the sedimentary system of Dongqiaoenbrak Formation. This transition from a turbidite system to a coastal alluvial fan represents a sedimentary response to the subduction-to-breakup dynamics along the margin of the Rodinia supercontinent.

Records of the accretionary, collisional and post-collisional evolution of western Gondwana in the high-grade core of the Araçuaí-Ribeira orogenic system, SE Brazil

High-grade metamorphism, extensive partial melting, and polyphase deformation often obliterate records of pre-collisional stages in ancient orogens, making it hard to unravel Precambrian accretionary events. In western Gondwana, the high-grade core of the Araçuaí-Ribeira Orogenic System (AROS) comprises pre-collisional rock-assemblages largely disrupted, overprinted, and remelted by collisional and post-collisional tectono-metamorphic and igneous events, hampering reconstructions of its accretionary history. Detailed studies reveled remnants of an Early Tonian (860–840 Ma) island arc, forming km-size roof-pendants and mega-xenoliths within the composite Caxixe batholith, in the boundary zone between the Araçuaí and Ribeira orogens. We further characterize the Caxixe batholith and surrounding plutons, showing that the main stage of magmatism is composed of pre-collisional orthogneisses, partially migmatized and crystallized between ca. 607–580 Ma, thus, much later than the Tonian island arc records. Those Ediacaran granitic rocks represent intrusions of calc-alkaline magmas with elemental geochemistry and isotopic (εHf(t): −6 to −10, whole-rock εNd(t): −4.9 to −9.5, and 87Sr/86Sr(i): 0.706–0.711) data correlated with the continental-margin Rio Doce arc. This contrasts with signatures indicative of a mantle source εHf(t) = +10 to + 14, εNd(t) = +0.9 to + 6.4, and 87Sr/86Sr(i) = 0.698–0.704) of the Early Tonian arc remnants. Intruding the pre-collisional granitic plutons, garnet-bearing leucogranites dated at ca. 575 Ma and a mafic intrusion of ca. 560 Ma mark the transition from the syn-collisional to late-collisional stages. Post-collisional, non-deformed, A-type granites with the most evolved isotopic signatures (εHf(t): −22; εNd(t): −5.6 to −13.2) intruded at ca. 500 Ma, ending the long-lasting orogenic history documented in the region. Our results demonstrate that a basement composed of an early Tonian juvenile arc docked with a continental margin and hosted several intrusions of an early Ediacaran continental-margin magmatic arc that, in turn, were intruded by collisional and post-collisional magmas from the Late Ediacaran to Cambrian, finally forming the main plutonic masses of the Araçuaí-Ribeira Orogenic System (AROS) core, during the complex history of western Gondwana assembly.

Tracking a magmatic arc within a confined orogen: New evidence from the Araçuaí orogen (SE Brazil)

By comparison with igneous rock assemblages of well-exposed modern magmatic arcs and their typical calc-alkaline, magnesian, I-type signatures, many deeply eroded magmatic arcs have been characterized worldwide, unraveling ancient plate margins. Since the 1980's, different assemblages of Ediacaran to Cambrian magmatic rocks have been systematically mapped and characterized according to their distinctive petrographic, structural, lithochemical, geochronologic, and isotopic features, allowing to establish the pre-collisional, collisional, and post-collisional stages of the Araçuaí orogen (SE Brazil). Among them, the tonalitic-granodioritic batholiths and stocks with calc-alkaline, magnesian, I-type signatures, rich in dioritic to mafic enclaves, and related metavolcano-sedimentary successions compose a magmatic arc formed in the early Ediacaran, the Rio Doce magmatic arc. Extending for more than 1000 km along the Araçuaí-Ribeira Orogenic System (AROS), the Rio Doce arc shows a hybrid isotopic signature and crustal inheritance, compatible with modern magmatic arcs built on continental active margins. We present new field, petrographic, and lithochemical studies, coupled with U–Pb (LA-ICPMS) geochronology and isotopic (in-zircon Lu–Hf and whole-rock Sm–Nd) analysis, unraveling the least evolved isotopic signatures ever found for tonalitic-granodioritic plutons of the Rio Doce magmatic arc, and a comprehensive data comparison with modern magmatic arcs. The studied Santa Tereza and Jequitibá suites comprise tonalitic-granodioritic and Opx-bearing charnockitic rocks, with calcic-to-alkali-calcic-magnesian I-type signature, associated with gabbroic rocks. Zircon U–Pb data yielded Concordia ages of 612 ± 3 Ma (Santa Tereza) and 594 ± 3 Ma (Jequitibá) for magmatic crystallization, and 580 ± 5 Ma to 571 ± 3 Ma for metamorphic recrystallization, with associated εNd(612Ma) values from −0.06 to −7.05 (TDM model ages: 1.4–2.04 Ga), and εNd(594Ma) values from −6.67 to −8.8 (TDM model ages: 1.64–1.87 Ga). The zircon εHf(t) (+2.3 to −4.8) and Hf TDM model ages (1.33–1.65 Ga) assigned a moderately juvenile to evolved signature for the Santa Tereza suite, while the Jequitibá suite (εHf(t): −1.7 to −36.4; TDM model ages: 1.33–3.21 Ga) shows a more evolved signature. Altogether, data from the Rio Doce arc are similar to typical continental-margin magmatic arcs with the involvement of mantle melts and/or melting of juvenile crust, suggesting a westward slab retreat during ocean-floor subduction within the Araçuaí confined orogen.

Geochemical characteristics and the constraints on paleoenvironment, provenance, and tectonic setting of Precambrian Xifangshan Formation in the northwestern Tarim Basin, NW China

Combining geochemistry and petrology to clarify paleoenvironment, provenance and tectonic setting is the most reliable approach. Few studies have been conducted on the Xifangshan Formation, a set of turbidite deposits from the Precambrian in the northwestern Tarim Basin, NW China. The tectonic setting during this period is not clear. This study reports the petrological characteristics and geochemical data of the Xifangshan sandstones, aiming to constrain the paleoclimate, paleoredox conditions, paleoweathering, provenance, and finally establish the tectonic development pattern of this period. Xifangshan Formation is mainly composed of litharenite and feldspathic litharenite. Most of the trace elements (Rb, Sr, Pb, U, Y, Sc, V, Cr, Cu, Co, Ni, Zn) are depleted relative to the upper continental crust. And, the rare earth elements show a slight negative Eu anomaly. The sandstones were first-cycle, displaying immaturity of composition and structure. The major element composition and low chemical index of alteration indicate that the sandstones have experienced weak weathering during transportation. Multiple geochemical indicators show that the Xifangshan Formation was deposited in an oxic-suboxic marine environment with moderate salinity and semiarid climate. Based on the geochemical and petrographic characteristics of the rock fragments, the source of the Xifangshan Formation has a felsic provenance, and the parent rock is predominantly acidic magmatic rocks, with a small amount of intermediate magmatic rocks. Influenced by the subduction of the ancient oceanic plate, the blueschist-bearing Aksu Group and the continental margin magmatic arc were formed. This magmatic arc provided the source for the Xifangshan Formation sandstones.

Evidence of late Palaeozoic and Middle Triassic magmatism in the Sakar-Strandzha Zone, SE Bulgaria: Regional geodynamic implications

ABSTRACT Late Palaeozoic granitoids, meta-granitoids and meta-volcanic rocks predominate in the metamorphic basement of the northern and western parts of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria, together with the subsidiary Triassic meta-granitoids and meta-volcanic rocks. Generally, igneous minerals and textures are preserved, except in the meta-granitoids and meta-volcanic rocks that experienced a low- to high-grade metamorphic overprint. The volcanic rocks have a peraluminous and high-K calc-alkaline composition, and the granitoids range between I- to S-type compositions, typical of volcanic arcs and syn-collisional settings. LILE and LREE-enrichment and Nb-Ta anomalies characterize the intrusive and extrusive rock suites. U-Pb zircon geochronology has yielded crystallization ages between 245 and 237 Ma for the majority of the studied igneous rocks, and between 297 and 295 Ma for a small group of igneous rocks. Early Permian and Middle Triassic igneous suites of the northern and western SASTZ have similar compositions and a similar tectonic setting when compared to Late Carboniferous-Early Permian intrusive and extrusive suites of the adjacent Sakar unit of the SASTZ, confirming a common regional late Palaeozoic-early Mesozoic tectono-magmatic event. As the Late Carboniferous-Permian to Middle Triassic magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Paleotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be linked with the Serbo-Macedonian-Rhodope zones to the southwest, where coeval meta-granitoids document the same geodynamic context. By contrast, the Triassic igneous suite of the SASTZ is unrelated to the Serbo-Macedonian-Rhodope zones, where Triassic meta-ophiolite and meta-granitoids record Neotethys rifting.

Neoproterozoic geodynamic evolution of easternmost Kalahari: Constraints from U-Pb-Hf-O zircon, Sm-Nd isotope and geochemical data from the Schirmacher Oasis, East Antarctica

Late Tonian (ca. 785–760 Ma) granodioritic to granitic orthogneisses of the Schirmacher Oasis region in Dronning Maud Land (DML), East Antarctica, are interpreted as recording an active continental margin setting at the periphery of Kalahari and Rodinia. The rocks probably represent exposures of a significant tectonic province hidden beneath the ice, the erosional remnants of which are recorded as detrital zircons in late Tonian-Cryogenian metasedimentary rocks throughout central and eastern DML, as well as in ice-rafted debris from recent sediments offshore Dronning Maud Land. The orthogneisses have single-stage Sm-Nd model ages of ca. 1.3–1.5 Ga and zircon Hf-signatures (εHft = +2 – +5), indistinguishable from the adjacent Grenville-age basement rocks of easternmost Kalahari. Their geochemistry suggests that they evolved in the late stages of a continental margin magmatic arc and possibly within a roll-back tectonic framework, suggestive of subduction of relatively old oceanic lithosphere. The eastern Kalahari continental arc is one of a number of continental arcs that characterize the western part of the fragmenting Rodinia and document the supercontinent “turning inside out” after its formation at ca. 1000 Ma and a period of relative tectonic quiescence between ca. 900 and 800 Ma.The rocks show an ultra-high temperature metamorphic overprint that was accompanied by syn-tectonic magmatism from ca. 650 to 600 Ma. The high temperature metamorphism is interpreted to relate to back-arc extension that also led to major anorthosite magmatism elsewhere, prior to continental collision in the region. The rocks lack the subsequent widespread high-grade metamorphic overprint at ca. 590–500 Ma which occurs in the adjacent regions due to Himalayan-style continental collision along the East African-Antarctic Orogen during Gondwana assembly.

Timing and tectonic significance of Paleozoic magmatism in the Sakar unit of the Sakar-Strandzha Zone, SE Bulgaria

ABSTRACTPalaeozoic granitoids and meta-granitoids dominate the metamorphic basement of the Sakar unit of the Sakar-Strandzha Zone (SASTZ) in southeast Bulgaria. In this article, we present new whole-rock geochemical data and U–Pb zircon geochronology for the Sakar unit granitoids. The igneous minerals and textures are preserved, except the meta-granitoids that experienced a weak amphibolite-facies overprint. Geochemistry reveals compositions of peraluminous high-K calc-alkaline I- to S-type granitoids of volcanic arc origin. A major group of LILE-LREE-enriched granitoids and meta-granitoids and a single HFSE-HREE-enriched meta-granitoid are distinguished. U–Pb geochronology has yielded crystallization ages between 305 and 295 Ma for the major group granitoids and a ca. 462 Ma crystallization age of HFSE-HREE-enriched meta-granitoid. Late Palaeozoic granitoids of the Sakar unit show similar compositions and a similar tectonic setting when compared to other granitoids of the SASTZ, confirming a uniform region-wide tectono-magmatic event. As the Late Carboniferous-Permian magmatic arc components extend across the SASTZ, they trace the time-correspondent active continental margin along the Eurasian plate during subduction of the Palaeotethys oceanic lithosphere. The late Palaeozoic Eurasian active continental margin magmatic arc evolution of the SASTZ can be extended into the Serbo-Macedonian-Rhodope zones to the west, where time equivalent meta-granitoids support the same geodynamic context.

An Andean-type retro-arc foreland system beneath northwest South China revealed by SINOPROBE profiling

In the Mesozoic, South China was situated along the convergent margin between the Asian and Pacific plates, providing an excellent laboratory to understand the interactions between deformation, sedimentation and magmatism in a retroarc environment. The crustal architecture of northwest South China is displayed along the ∼600-km-long SINOPROBE deep seismic reflection profiles and reveals from east to west: (1) highly folded and truncated reflectors in the upper crust of the Yangtze Fold Zone, which correspond to thin- and thick-skinned thrust systems, and document large-scale intraplate structural imbrication and shortening; (2) a crustal-scale flat-ramp-flat structure, termed the Main Yangtze decollement, which forms a weak, viscous layer to accommodate strain decoupling and material transport in the thin- and thick-skinned systems; and (3) nearly flat-lying reflectors in the Sichuan Basin, which support interpretation of the basin as a weakly deformed depocentre. The Yangtze Fold Zone and the Sichuan Basin represent a retro-arc foreland basin system that is >800 km away from the continental-margin magmatic arc. We suggest that tectonic processes across the arc and retro-arc systems, including arc magma flare-up, basin sedimentation, retroarc thrust propagation, lithosphere underthrusting, root foundering, and extension-related magmatism were interrelated and governed mass transfer. Age data and geological relations link the tectonic processes to evolving geodynamics of the subducting Paleo-Pacific plate.

Composition and age of Jurassic diabase dikes in the Xiongcun porphyry copper–gold district, southern margin of the Lhasa terrane, Tibet, China: Petrogenesis and tectonic setting

Diabase dikes are extensively distributed in the Xiongcun porphyry copper–gold district in the southern margin of the Lhasa terrane. Here, we report zircon U–Pb ages determined by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) along with Hf isotopic, whole‐rock elemental, and Sr–Nd–Pb isotopic data for these diabase dikes. Zircons from one of the Xiongcun diabase dikes yielded an emplacement age of 165.3 ± 1.0 Ma. The Xiongcun diabase dikes display subalkaline features and tholeiite compositions. They are enriched in large‐ion lithophile elements (e.g., Rb, Ba, and K) and light rare earth elements and are depleted in high‐field‐strength elements (e.g., Nb, Ta, and Ti). The Xiongcun diabase dikes exhibit relatively low ratios of (87Sr/86Sr)i(0.70386–0.70501), (206Pb/204Pb)i(18.15–18.40), (207Pb/204P)i(15.52–15.57), and (208Pb/204Pb)i(38.06–39.46) and relatively high values of εNd(t) (5.85–6.92) and εHf(t) (+12.58 to +14.03). Combined with previous research results, these findings suggest that the Xiongcun diabase dikes and Early Mesozoic magmatic rocks in the southern margin of the Lhasa terrane formed in an active continental margin magmatic arc setting related to the northward subduction of the Neo‐Tethys oceanic slab; however, the possibility that the Xiongcun diabase dikes were formed in an intra‐oceanic arc setting cannot be excluded. Their parental magmas were formed by the partial melting of the depleted mantle source that was modified by fluids released from the Neo‐Tethys oceanic slab. The magmas experienced fractional crystallization of olivine and clinopyroxene and underwent limited crustal contamination during magma emplacement. The ~165 Ma Xiongcun diabase dikes, which probably originated under an extensional stress condition in a magmatic arc setting, correspond to the timing of Middle Jurassic subduction‐related porphyry copper mineralization (161–172 Ma) in the southern margin of the Lhasa terrane, likely suggesting that this episode of mineralization occurred during a period of changing stress from compression to extension.

Recognizing Non-Steady-State Weathering Conditions In Ancient Submarine Fan Systems: Correcting For K-Metasomatism Reveals A Late Cretaceous Non-Steady-State Weathering Regime, Point Loma Formation, Southwestern California, U.S.A.

Abstract: During steady-state weathering, the rate of removal of the exposed weathering zone is balanced by the rate of generation of mineralogically identical material through chemical weathering. As steady-state conditions prevail, the mineralogical and chemical composition of sediments derived from the weathering profile should be invariant over time. In A-CN-K space, a steady-state weathering condition is characterized by tightly grouped compositions, with dispersion between sands and muds a consequence of sorting during fluvial transportation. The purpose of this study was to assess whether such a condition was present during the development of the Upper Cretaceous Point Loma Formation. Published paleotectonic reconstructions portray the upper Campanian–lower Maastrichtian(?) Point Loma Formation as being deposited in a forearc basin developed westward of a continental-margin magmatic arc. In such accounts, during the subduction of an oceanic plateau (or aseismic ridge) or during constant-dip subduction, rocks in the magmatic arc were uplifted and the volcanic cover nearly stripped clean, exposing its plutonic roots. Published detrital-zircon data derived from samples of the Point Loma Formation indicate that exposed igneous rocks in the source area were mainly from 135 Ma to 100 Ma plutons located in the western zone of the Peninsular Ranges batholith. New point-count, XRD, and chemical data show that detritus in mudstones and some fine-grained feldspathic sandstones of the Point Loma Formation following deposition and shallow burial were subjected to a K-metasomatic event. After correcting for introduction of K + , and consideration of the effects of hydraulic sorting, resulting mudstone and fine-grained sandstone data plot in A-CN-K space in overlapping fields, whereas samples of coarse-grained sandstone lie between rays extending through the most and least weathered mudstones. Hence, detritus in the Point Loma Formation was derived mainly from regolith during non-steady-state weathering conditions.

U-Pb ages and geochemistry of zircon from Proterozoic plutons of the Sawatch and Mosquito ranges, Colorado, U.S.A.: Implications for crustal growth of the central Colorado province

A broad study of zircons from plutonic rocks of the Sawatch and Mosquito ranges of west-central Colorado (U.S.A.) was undertaken to significantly refine the magmatic chronology and chemistry of this under-studied region of the Colorado province. This region was chosen because it lies just to the north of the suspected arc-related Gunnison-Salida volcano-plutonic terrane, which has been the subject of many recent investigations—and whose origin is still debated. Our new results provide important insights into the processes active during Proterozoic crustal evolution in this region, and they have important ramifications for broader-scope crustal evolution models for southwestern North America. Twenty-four new U-Pb ages and sequentially acquired rare-earth element (REE), U, Th, and Hf contents of zircon have been determined using the sensitive high-resolution ion microprobe-reverse geometry (SHRIMP-RG). These zircon geochemistry data, in conjunction with whole-rock major- and trace-element data, provide important insights into zircon crystallization and melt fractionation, and they help to further constrain the tectonic environment of magma generation. Our detailed zircon and whole-rock data support the following three interpretations: (1) The Roosevelt Granite in the southern Sawatch Range was the oldest rock dated at 1,766 ± 7 Ma, and it intruded various metavolcanic and metasedimentary rocks. Geochemistry of both whole-rock and zircon supports the contention that this granite was produced in a magmatic arc environment and, therefore, is likely an extension of the older Dubois Greenstone Belt of the Gunnison Igneous Complex (GIC) and the Needle Mountains (1,770–1,755 Ma). Rocks of the younger Cochetopa succession of the GIC, the Salida Greenstone Belt, and the Sangre de Cristo Mountains (1,740–1,725 Ma) were not found in the Sawatch and Mosquito ranges. This observation strongly suggests that the northern edge of the Gunnison-Salida arc terrane underlies the southern portion of the Sawatch and Mosquito ranges. (2) Calc-alkalic to alkali-calcic magmas intruded this region approximately 55 m.y. after the Roosevelt Granite with emplacement of pre-deformational plutons at ca. 1,710 Ma (e.g., Henry Mountain Granite and diorite of Denny Creek), and this continued for at least 30 m.y., ending with emplacement of post-deformational plutons at ca. 1,680 Ma (e.g., Kroenke Granodiorite, granite of Fairview Peak, and syenite of Mount Yale). The timing of deformation can be constrained to sometime after intrusion of the diorite of Denny Creek and likely before the emplacement of the undeformed granite of Fairview Peak. Geochemistry of both whole-rock and zircon indicates that the older group of ca. 1,710-Ma plutons formed at shallower depths, and then they intruded the younger group of more deeply generated, commonly peraluminous and sodic plutons. Although absent in the Sawatch and Mosquito ranges, Mazatzal-age (ca. 1,680–1,620 Ma) plutonic rocks are present regionally. Inherited zircon components of Mazatzal-age were found as cores in some 1.4-Ga Sawatch and Mosquito Range zircons, indicating the likelihood of a relatively local source. These combined data suggest the possibility that all were produced within a continental-margin magmatic arc created as a result of southward-migrating (slab rollback?), north-dipping subduction to the south of the region. (3) Widespread Mesoproterozoic plutonism—with emplacement at various depths and exhibiting bimodal geochemistry—is recognized in 16 different samples. An older group of predominantly peraluminous, yet magnesian granitoids (e.g., granodiorite of Sayers, granite of Taylor River, and the St. Kevin Granite) were emplaced between ca. 1,450 and 1,425 Ma. These geochemical parameters suggest moderate degrees of partial melting in a low-pressure environment. Three younger metaluminous, but ferroan plutons (diorite of Grottos, diorite of Mount Elbert, and granodiorite of Mount Harvard), probably represent a final magmatic pulse at ca. 1,416 Ma. A comprehensive treatment of zircon REE and whole-rock trace-element behavior from Proterozoic rocks is scarce. Discriminant U/Yb versus Y diagrams using zircon data show that the Sawatch and Mosquito plutons are of continental origin, not oceanic. Additional bivariate diagrams incorporating cation ratio combinations of Gd, Ce, Yb, U, Th, Hf, and Eu offer refined insight into differences in fractionation trends and depth of magma generation for the various plutons. These interpretations, on the basis of zircon trace-element data, are mirrored in the whole-rock geochemistry data.

Discovery and Geological Significance of Neoproterozoic Metamorphic Granite in Jimo, Shandong Province, Eastern China

AbstractDuring the 1:50000 regional geological survey in Jimo, east Shandong Province, Paleoproterozoic metamorphic supracrustal rocks and Neoproterozoic metamorphic plutonite were newly discovered. These rocks displayed inclusions which had occurred in the Mesozoic granite, and the main lithologies are schist, granulite, marble, and granitic gneiss. Geochemical analyses suggest that Neoproterozoic metamorphic plutonite are characterized by high‐K, metaluminous to weakly peraluminous. They are enriched in LILE and depleted in HFSE, with moderately enrichment of LREE, weak fractionation of LREE from HREE and negative Eu anomalies. The surface age of plutonic rocks in the survey area is 770.2±2.4 Ma, representing the age of magma crystallization, which is agreement with the the Neoproterozoic magmatic event after Rodinia supercontinent in the northern margin of Southern China continental block. In addition, the age of sporadic distribution (298 Ma and 269 Ma) is mixed zircon age, representing the rocks experienced metamorphism in Indosinian period. According to the associated mineral assemblages, and the characteristic metamorphic minerals and temperature pressure conditions, four metamorphic facies were identified, including amphibolitic, epidote amphibolite, greenschist, and mid‐high pressure greenschist. Analysis of tectonic setting suggests that granitic gneiss is formed in an extensional environment and was involved from the continental margin magmatic arc to intraplate environment. Jimo is distributed in the east of Zhuwu fault, and has the same Spatial distribution location with the Weihai uplift UHP metamorphic belt rocks. The metamorphic rocks in Jimo area have similar geochemical characteristics of elements, tectonic setting and retrograde metamorphism with that in the Sulu UHP metamorphic belt. Therefore, Zhuwu fault may be the boundary fault of Sulu UHP metamorphic belt.

Discovery, Geology, and Origin of the Fruta del Norte Epithermal Gold-Silver Deposit, Southeastern Ecuador

Fruta del Norte is a completely concealed and extraordinarily well preserved epithermal gold-silver deposit located in the remote Cordillera del Condor mountain ranges of southeastern Ecuador. Currently defined resources in a single, steep, relatively small body, displaying exceptional grade continuity, contain 9.81 million ounces (Moz) of gold and 15.0 Moz of silver, with an indicated resource grade of 9.59 g/t Au and 12.9 g/t Ag. The deposit was discovered in early 2006 during a greenfields program conducted by Aurelian Resources, a Canadian junior explorer. Discovery resulted from systematic drill testing of a conceptual geologic model, which predicted that auriferous veins would be present in andesitic volcanic rocks inferred to underlie a steep silicified rib cutting a fluvial conglomerate sequence. The rib is highly anomalous in arsenic, antimony, and mercury but contains low-order gold values. The second drill test—the discovery hole—intersected >230 m of ore-grade gold-silver mineralization beneath ~200 m of the barren conglomerate cover. The host andesitic volcanic rocks, crosscutting feldspar porphyry, and associated phreatic breccias are assigned to the Piuntza unit of the Santiago Formation, which, along with the Zamora batholith and a series of porphyry copper stocks, was generated in a continental margin magmatic arc during the Middle to Late Jurassic. The deposit is located near the northeastern extremity of the ~10-km-long, Suarez pull-apart basin where it is linked to the steep West and Central faults, part of the regionally extensive Las Penas strike-slip fault zone. The pull-apart basin was progressively filled by fluviatile conglomerate, dacitic ignimbrite flows, finer grained siliciclastic sedimentary rocks, and, finally, andesite flows, all assigned to the Chapiza Formation. The Fruta del Norte deposit consists of a 1.3-km-long and up to >300-m-wide vein-stockwork associated with quartz-illite-pyrite alteration. The deposit comprises two principal vein types, one in the south dominated by quartz, manganoan carbonates, and abundant base metal sulfides and the other in the north dominated by manganese- and base metal-poor quartz and calcite. Adularia is a minor main-stage gangue mineral in each. Both types are abruptly transitional upward and westward to a third ore type marked by intense silicification and chalcedony veining, with disseminated and veinlet marcasite (±pyrite). The uppermost part of this silicic zone is sulfide deficient, probably the result of a short-lived supergene oxidation event prior to initial conglomerate deposition. The deposit is notable for the widespread occurrence of fine to coarse visible gold, which gives rise to bonanza grades and is closely associated with quartz, chalcedony, carbonate, and sulfide gangue. The sulfide-free, silicic zone is overlain by an extensive silica sinter horizon, which may either directly overlie the Piuntza volcanic rocks and/or occur as interbeds in the basal 20 m of the conglomerate, which are invariably silicified and marcasite bearing. Otherwise, the conglomerate above the orebody lacks silicification, with one critical exception: the steeply inclined zone that is exposed as the silicified rib that led to discovery. The sinter horizon, containing localized mud-pool deposits and hydrothermal eruption breccias, is in unusual proximity to the underlying gold-silver orebody. The northern and southern parts of the Fruta del Norte deposit possess characteristics that are usually considered typical of low and intermediate sulfidation epithermal deposits, respectively; they may have required two discrete mineralizing fluids, both of which are suspected to have ascended via the West and Central faults. The upward change to the silicified zone is attributed to cooling of the ascendant fluid on approach to the paleosurface sinter. Progressive burial of the deposit beneath the Suarez basin fill may have contributed to the suppression and eventual extinction of the epithermal system, with gold mineralization of the conglomerate being inhibited by rheologic and chemical factors. Formation of the Fruta del Norte deposit and complete filling of the Suarez basin took place in >3 to 6 m.y. during the Late Jurassic ( 157–154 Ma).

Residual regolith derived from the biotite-controlled weathering of Cretaceous tonalite–quartz diorite, Peninsular Ranges, southern California, USA: A case study

Though K-feldspar-free tonalite and quartz diorite are common plutonic rocks in exhumed continental-margin magmatic arc rocks around the globe, how they weather has not been extensively studied. We therefore undertook an investigation aimed at understanding how an approximately 6-m thick section of residual regolith (saprock and overlying Alfisol) developed from Cretaceous K-feldspar-free tonalite and quartz diorite at Santa Margarita Ecological Reserve, southern California, USA. We analyzed three samples of soil, 10 of saprock, and 10 of corestone for their major, trace, and rare earth element compositions. Thin sections of each major textural group also were studied and point counted. Common XRD techniques were used to determine clay mineralogy. Three samples of soil and five of saprock were analyzed for their pH.Within the saprock, an extensive network of cracks was likely produced by the volumetric expansion that accompanied the conversion of biotite to vermiculite. Migrating near neutral fluids responsible for this conversion resulted in a loss of K mass and affected several trace elements that are commonly found in biotite, but with the exception of an interval between 177 and 270cm, left most other major, trace, and rare earth elements (REE) little affected. Within the 177 to 270cm interval, at a depth of 208cm, relative to the immobile framework, incongruent leaching of plagioclase by acidic solutions, resulted in the losses of Si, Al, Ca, Na, Pb, Sr, and Ga mass. In contrast, more neutral solutions added heavy REE (HREE) and Y mass at 177 to 237cm depths, while acidic solutions removed REE and Y mass at 270cm depth. In the soil, acidic solutions attacked plagioclase, biotite, and apatite leaching and removing, relative to the immobile framework, Ca, Na, Mg, K, and P mass. In addition, the dissolution of apatite resulted in depletions in HREE and Y mass. None of the elemental mass removed from the soil accumulated in the directly underlying saprock, and must have been removed from the near vertical plane of sampling.Within the saprock, the above changes are mostly the result of near neutral pH water/rock interactions occurring within a warm and dry Holocene climate. Identified changes in elemental mass, other than those associated with the conversion of biotite to vermiculite, are localized and are likely the result of the paths taken by fluids with differing pH moving through the complex network of cracks. Any model purporting to describe the evolution of residual regolith must explain such natural complexities.

Tonian granitic magmatism of the Borborema Province, NE Brazil: A review

Tonian granitoids, today augen-gneisses and migmatites, showing crystallization ages ranging from 870 Ma to 1000 Ma occur in the Borborema Province, NE Brazil. The majority of them have ages within the 925–970 Ma interval. Few intrusions with ages of ∼1.0 Ga and <900 Ma occur in the Transversal and South subprovinces. The Tonian granitoids constitute the most expressive magmatic rocks of the Cariris Velhos event. The studied granitoids (herein CVG -Cariris Velhos granitoids) intrude slightly older bimodal (but mostly felsic) volcanic successions and metasedimentary sequences in the Transversal and South subprovinces. Tonian granitoids are unknown in the North subprovince. The CVG comprise mainly coarse-grained augen-gneisses of granite to granodiorite composition. Fe-rich biotite (annite) is the main mafic mineral phase, constituting up to 15% of the modal composition. Garnet, muscovite and tourmaline occur as accessory phases in many plutons.The CVG augen–gneisses have high SiO2 (>71%) and alkali contents, they vary from slightly peraluminous to slightly metaluminous, and from slightly magnesian to typical ferrroan rocks. In the migmatized orthogneisses the SiO2 contents are usually <70%. Trace element variations in the CVG are extensive, reflecting the migmatization recorded in some plutons and/or distinct sources. They are Ca-, Sr- and Nb-poor, showing variable Ba (100–1260 ppm), Rb (164–400 ppm) and Zr (144–408 ppm) contents, and high abundances of Y (>40 ppm). The chondrite normalized REE patterns are characterized by strong to moderate negative Eu anomalies (Eu/Eu* = 0.23–0.70). In general, the spidergram patterns show deep troughs at Ti, P, Ba and Sr and less pronounced Nb–Ta troughs. These patterns are similar to those reported for anorogenic granites evolved from mixtures of magmas from both crustal and mantle sources. The CVG exhibit TDM model ages ranging from 1.9 to 1.1Ga, with slightly negative to slightly positive ƐNd(t) values, suggesting the involvement of distinct proportions of mantle and crustal components in the source of their protoliths.There is no consensus in the literature about the tectonic setting of the CVG ie they have been related to either continental margin magmatic arc, with possible back–arc association, or extention-related setting, with generation of A-type granites. However, all the available geochemical data suggest that the CVG represent extension related magmatism. The geochemical signature associated to bimodal volcanism, including pyroclastic rocks, with similar ages, and absence, up to now, of evidence for metamorphism of Tonian age, support the hypothesis of extension - related magmatism.

Detrital-zircon geochronology and provenance of the Ocloyic synorogenic clastic wedge, and Ordovician accretion of the Argentine Precordillera terrane

The Precordillera terrane in northwestern Argentina is interpreted to be an exotic (Laurentian) continental fragment that was accreted to western Gondwana during the Ordovician. One prominent manifestation of the subduction and collision process is a Middle–Upper Ordovician clastic wedge, which overlies a passive-margin carbonate-platform succession in the Precordillera. U/Pb ages of detrital zircons from sandstones within the clastic wedge, as well as zircons from clasts within conglomerates, provide documentation for the composition of the sediment provenance. The ages of detrital zircons are consistent vertically through the succession, as well as laterally along and across strike of the Precordillera, indicating a single, persistent sediment source throughout deposition of the clastic wedge. The dominant mode (∼1350–1000 Ma) of the detrital-zircon ages corresponds to the ages of basement rocks in the Western Sierras Pampeanas along the eastern side of the Precordillera. A secondary mode (1500–1350 Ma) corresponds in age to the Granite-Rhyolite province of Laurentia, an age range which is not known in ages of basement rocks of the Western Sierras Pampeanas; however, detritus from Granite-Rhyolite-age rocks in the basement of the Precordillera was available through recycling of synrift and passive-margin cover strata. Igneous clasts in the conglomerates have ages (647–614 Ma) that correspond to the ages of minor synrift igneous rocks in the nearby basement massifs; the same ages are represented in a minor mode (∼750–570 Ma) of detrital-zircon ages. A quartzite clast in a conglomerate, as well as parts of the population of detrital zircons, indicates the importance of a source in the metasedimentary cover of the leading edge of the Precordillera. The Famatina continental-margin magmatic arc reflects pre-collision subduction of Precordillera lithosphere beneath the western Gondwana margin; however, no detrital zircons have ages that correspond to Famatina arc magmatism, indicating that sedimentary detritus from the arc may have been trapped in a forearc basin and did not reach the foreland. The indicators of sedimentary provenance for the foreland deposits are consistent with subduction of the Precordillera beneath western Gondwana, imbrication of basement rocks from either the Precordillera or Gondwana into an accretionary complex, and recycling of deformed Precordillera cover rocks.

Mesoproterozoic continental arc magmatism and crustal growth in the eastern Central Tianshan Arc Terrane of the southern Central Asian Orogenic Belt: Geochronological and geochemical evidence

Numerous microcontinents are known to occur in the Central Asian Orogenic Belt (CAOB), one of the largest accretionary orogens and the most significant area of Paleozoic crustal growth in the world. The evolution of the Precambrian crust in these microcontinents is central to understanding the accretionary and collisional tectonics of the CAOB. Here, we present systematic zircon U–Pb dating and Hf isotope studies of Mesoproterozoic gneissic granitoids from the eastern Central Tianshan Arc Terrane (CTA) of the southern CAOB. The investigated intermediate to felsic (SiO2=60.48–78.92wt.%) granitoids belong to the calcic- to calc-alkaline series and usually have pronounced negative Nb, Ta and Ti anomalies, relative enrichments of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) while heavy rare earth elements (HREEs) and high field strength elements (HFSEs) are depleted, revealing typical active continental margin magmatic arc geochemical characteristics. These spatially-distant rocks show consistent zircon U–Pb crystallization ages from ca. 1.45 to 1.40Ga and thus constitute a previously unknown Mesoproterozoic continental magmatic arc covering hundreds of kilometers in the eastern segment of the CTA. Furthermore the high and mainly positive zircon εHf(t) values between −1.0 and +8.6 and the zircon Hf model ages of 1.95 to 1.55Ga, which are slightly older than their crystallization ages, suggest that they were mainly derived from rapid reworking of juvenile material with a limited input of an ancient crustal component. Therefore, the formation of these granitoids defines an extensive Mesoproterozoic intermediate to felsic, subduction-related intrusive magmatic arc activity that was active from at least 1.45 to 1.40Ga, involving significant juvenile continental growth in the eastern segment of the CTA. Furthermore the zircon U–Pb and Hf isotopic data challenge the common belief that the CTA was part of the Tarim Craton during Paleo- to Mesoproterozoic times. However, the southern Beishan microcontinents and the CTA have similar zircon U–Pb and Hf isotopic signatures, indicating a similar Precambrian tectonothermal evolution for both. This finding provides new and important constraints on the Precambrian crustal evolution of the microcontinents in the southern CAOB.

The Afeição augen-gneiss Suite and the record of the Cariris Velhos Orogeny (1000–960 Ma) within the Riacho do Pontal fold belt, NE Brazil

Early Tonian (∼1000–920 Ma) rocks occur within the Transversal Zone of the Borborema Province in Northeast Brazil comprising the 700 km-long sigmoidal Cariris Velhos belt. The Afeição augen-gneiss Suite crops out in the internal zone of the Riacho do Pontal fold belt, about 100 km southwestward of the closest Cariris Velhos occurrence within the Transversal Zone, and has been proposed to represent a continuation of this belt within the Southern subprovince of the Borborema Province. Several plutons included within this unit intrude or are thrust upon metavolcanosedimentary sequences of three different units (Santa Filomena, Paulistana and Morro Branco complexes). The Afeição Suite is composed mostly of calc-alkaline, high-K, peraluminous, high-silica ferroan and magnesian granites. Chondrite-normalized REE patterns are moderate to highly fractionated, with a pronounced negative Eu anomaly. Incompatible element spidergrams show a negative Nb–Ta anomaly, akin to convergence setting (Cordilleran-type) granites. U–Pb zircon data constrain the age of crystallization between 1000 and 960 Ma, thus confirming chrono-correlation with the Cariris Velhos belt. Values of εNd(t) between −1.0 and +3.1 and TDM of 1.2–1.5 Ga, similar to other Cariris Velhos occurrences, suggest variable mixing of Tonian juvenile sources with older crustal sources, the latter involving Archean/Paleoproterozoic basement. Although there are currently no reliable geochronological data for the supracrustal sequences of the internal zone, cross-cutting relationships indicate that the Santa Filomena and Morro Branco complexes are older or of similar age to the Afeição Suite, and thus could be related to the Cariris Velhos Orogeny. Based on petrographic, lithogeochemical, geochronological and isotope data, and according to previous models proposed for the Cariris Velhos belt, we interpret the Afeição Suite as the southwestern edge of a continental margin magmatic arc accreted to this portion of West Gondwana during the Early Tonian. Late-Brasiliano dextral displacement through the western branch of the Pernambuco shear zone separated these Cariris Velhos occurrences from its equivalents within the Transversal Zone of the Borborema Province (Recanto-type augen-gneiss of the Alto Pajeú Terrane).

Geochemistry of paleozoic terrigenous sediments from the Oldoi terrane, eastern Central Asian orogenic belt, as an indicator of geodynamic conditions during deposition

The analysis of the major and trace element geochemistry of Paleozoic terrigenous sediments from the Oldoi terrane, eastern Central Asian orogenic belt, supports a predominantly felsic source consisting of granites and recycled sediments. The geological data suggest that detrital material could be derived from Early Paleozoic granitoids, which were identified within tectonic blocks in the Silurian and Devonian terrigenous successions. The analysis of conventional discrimination diagrams indicates that the initial stage was marked by deposition along a passive continental margin, which continued in island-arc or active continental margin environments. This interpretation is consistent with geological evidence, such as the presence of volcanic interlayers and lenses in the Middle-Late Devonian successions and granitoids with ages of 386 +/- 10 and 371+/- 5.5 Ma. The timing of changes in geodynamic settings constrained by tectonic reconstructions corresponds to the age of the inception of the Norovlya continental-margin magmatic arc.