Continental Magmatic Arc Research Articles

Toward an integrated model of geological evolution for NE Brazil-NW Africa: The Borborema Province and its connections to the Trans-Saharan (Benino-Nigerian and Tuareg shields) and Central African orogens

Both the Borborema Province of NE Brazil and the geological provinces of NW Africa (the Trans-Saharan Orogen consisted of the Tuareg and Benino-Nigerian shields and the Central African Orogen of Cameroon, Chad, and Central African Republic) are complex geological regions with superposition of distinct deformational, metamorphic and magmatic events and final structural configuration during the Brasiliano/Pan-African Orogeny (ca. 625-510 Ma). These provinces represent the site of major mountain building processes in the Ediacaran/Cambrian transition that culminated in the amalgamation of West Gondwana after the collision of the West African-São Luís, São Francisco-Congo, and Saharan paleocontinents. In the last years, discovery and characterization of key tectonic units such as ophiolites, eclogites, HP/UHP rocks, and both oceanic and continental magmatic arcs are helping to clarify these processes and propose tectonic models for the geological evolution of NE Brazil-NW Africa. Connections of the marginal belts that frame these provinces, bordering the eastern margin of the West African-São Luís Craton (Médio Coreaú-Dahomeyides-Gourma-West Tuareg Shield) and the northern margin of the São Francisco-Congo Craton (Rio Preto-Riacho do Pontal-Sergipano-Yaoundé-Central African) are progressively better constrained, while correlations within the interior, highly reworked and sectioned portions of both the Borborema Province, the Benino-Nigerian Shield, the Central and East Tuareg Shield, Western Cameroon, and Adamawa-Yadé domains are more complicated and demand further investigation. Some of the questions of prime importance in this context are the continuation or not of the 1000-920 Ma Cariris Velhos Belt of NE Brazil into NW Africa, and if the basement-dominated North Borborema/Benino-Nigerian (NOBO-BENI) and Alto Pajeú-Alto Moxotó-Rio Capibaribe-Pernambuco-Alagoas/Adamawa-Yade (APAMCAPAY) domains could represent major decratonized blocks (such as LATEA in the Central Tuareg Shield), perhaps developed due to hyperextension and detachment of a Greater São Francisco-Congo paleocontinent northern margin. In this case, the Goiás-Pharusian and Transnordestino-Central African oceanic realms along with restricted internal oceans such as the hypothetical Piancó-Alto Brígida/Western Cameroon (PAB-WECA) Seaway probably separated these ancient paleocontinental blocks during the Neoproterozoic. The development of subduction zones and the docking of Neoproterozoic juvenile terranes welded the hyperextended Archean/Paleoproterozoic lithospheric fragments together and they became squeezed and reworked in between the major cratonic landmasses during the Brasiliano/Pan-African Orogeny. The quest for the sites of ancient oceans and continents that once composed NE Brazil and NW Africa goes on and tentative scenarios will surely benefit from novel geological, isotopic, and geochronological data put forward in the near future.

Chapter 8 Outboard-migrating accretionary orogeny at 1.9–1.8 Ga (Svecokarelian) along a margin to the continent Fennoscandia

Abstract An intimate lithostratigraphic and lithodemic connection between syn-orogenic rock masses inside the different lithotectonic units of the 2.0–1.8 Ga (Svecokarelian) orogen, Sweden, is proposed. A repetitive cyclic tectonic evolution occurred during the time period c. 1.91–1.75 Ga, each cycle lasting about 50–55 million years. Volcanic rocks ( c. 1.91–1.88 Ga) belonging to the earliest cycle are host to most of the base metal sulphide and Fe oxide deposits inside the orogen. Preservation of relict trails of continental magmatic arcs and intra-arc basins is inferred, with differences in the depth of basin deposition controlling, for example, contrasting types of base metal sulphide deposits along different trails. The segmented geometry of these continental magmatic arcs and intra-arc basins is related to strike-slip movement along ductile shear zones during transpressive events around and after 1.88 Ga; late orogenic folding also disturbed their orientation on a regional scale. A linear northwesterly orogenic trend is suggested prior to this structural overprint, the strike-slip movement being mainly parallel to the orogen. A solely accretionary orogenic model along an active margin to the continent Fennoscandia, without any trace of a terminal continent–continent collision, is preferred. Alternating retreating and advancing subduction modes that migrated progressively outboard and southwestwards in time account for the tectonic cycles.

Petrogenesis of the Riacho do Icó Stock: evidence for Neoproterozoic slab melting during accretion tectonics in the Borborema Province?

Abstract One of the main characteristics of the Borborema Province in northeastern Brazil is the abundance of granitic domains. The Lagoa das Pedras plutonic-volcanic Complex record remnants of early to late Neoproterozoic primitive to highly evolved magmas. U-Pb SHRIMP determinations and whole-rock geochemistry on the granitic to granodioritic Riacho do Ico Stock, which is the largest plutonic body of this complex, were conducted. Zircon dating reveals that this body was intruded in the crust at 607 ± 3 Ma, whereas metamorphic overgrowths are coeval within uncertainty (600 ± 8 Ma). Geochemical characteristics of this stock suggest that it corresponds to Cordilleran-type magmas injected in the lithosphere above a subduction zone. Sr and Y contents in addition to MgO and SiO2 values are compatible with high-silica adakite-like magmas, interpreted as the result of slab melting in deep-seated regions. Based on the integration of this study and literature data, we suggest that the Riacho do Ico intrusion might be a dismembered part of a major continental magmatic arc described in the Central Subprovince of the Borborema Province, marking the onset of the accretionary stage of the Brasiliano orogeny.

Geology, geochronology Pb-Pb, U-Pb-Hf zircon and Sm-Nd TDM of the Uruburetama batholith, Northern Borborema Province: contextualization in the Santa Quitéria Magmatic Arc

Records of large crustal masses in the northwest of Borborema Province (BP) spread over more than 300 km are found as numerous granitic bodies amid a high-grade metamorphic gneissic-migmatitic terrain. The Tamboril-Santa Quitéria Complex (TSQC) is a Neoproterozoic example of these records; it is located in the north of the Ceará Central Domain (CECD) and its origin is related to a continental magmatic arc at different evolutionary stages of the arc. The Uruburetama Granite (UG), object of this study, fits into this context and constitutes one of the most representative batholiths occupying an area of 1,500 km2. Dozens of other similar plutons have been recorded in cartographic works that show similarities which suggests the grouping of these bodies in the Uruburetama Granitic Suite (UGS), previously included in the TSQC. The UG consists of a variety of plutonic rocks associated with mafic-dioritic dikes and its metamorphic products. Six petrographic facies were identified among the plutonic rocks: biotite-hornblende monzogranites and syenogranitic varieties, biotite-hornblende granodiorites, in addition to quartz syenite, leucomonzogranites, and diorites rocks. These rocks are affected by deformation, with greater intensity at the edges of the batholith, where thrust shear zones with transcurrent components were installed, generating tectonic fabrics (mylonitic foliations, stretch lineation, almond feldspar porphyroclasts, and ribbon quartz). Despite the superimposed deformational effects, magmatic relict textures are partially preserved, mainly toward the central portion of the pluton. The UG and the associated plutons are hosted by migmatized orthogneisses, paragneisses with garnet, sillimanite or kyanite, marbles, calcium-silicate gneisses, micaschists, sillimanite quartzites, and amphibolite lenses. The contact relations are diffuse with gradation for migmatites, and in rare cases, intrusive contacts with gneiss mega-xenolites are registered. The geochronological data obtained by Pb-evaporation zircon in two samples of the UG show average ages of 655 ± 2 and 656 ± 1 Ma. U-Pb zircon data for the same samples showed slightly younger age values of 559 ± 10 and 634 ± 10 Ma, respectively. The ages obtained are considered as representative of the magmatic phases of UG crystallization in the context of the evolution of the Santa Quitéria Magmatic Arc (SQMA), with the main magmatic phase of the UG in the Cryogenian. However, the age of 559 Ma would represent a younger magmatic event in the evolution of the arch. Overall, these ages correspond to the “Pre-Collisional I” or “Early-Sin-Orogenic” phase of a large collisional belt in the west of BP, and the younger ages must represent events related to the continuity of the orogen convergence of the Latest-Pre-collisional I (634 Ma) and Post-Collisional I (559 Ma) phases. Sm-Nd whole-rock isotopes data showed εNd(t) values predominantly negative (-25.6 and −0.9), and Nd-TDM model ages of 2.90 and 1.2 Ga. The results of the Lu–Hf isotopes analysis for the Bt-Hbl monzogranite sample showed negative εHf values (-26.75 to −35.48) and Hf-TDMC model ages of 3.12 to 3.65 Ga while the results for Bt-Hbl granodiorite showed εHf values of −2.07 to +1.08 and Hf-TDMC model ages of 1.46 to 1.66 Ga. These data point to the existence of two distinct and older crustal sources in the generation of these granitoids, one in the Mesoarchean, and the other in the Mesoproterozoic. The Archean ages correspond to the basement in the south of the CECD (Tróia Massif and Granjeiro Complex); on the other hand, the Mesoproterozoic ages are up for discussion, since terrains with this type of age do not occur adjacent to the BP. On the other hand, it can be interpreted as a mixture of sources, one of them probably juvenile, neoproterozoic, with contamination of the Archean crust. Thus, the UG is considered one of the most important records of the beginning of the evolution of the Santa Quitéria Magmatic Arc associated with a wide collisional belt in the “West Gondwana Orogen” in the west of Borborema Province.

U–Pb Age and Hf Isotope Geochemistry of Detrital Zircons from Cambrian Sandstones of the Severnaya Zemlya Archipelago and Northern Taimyr (Russian High Arctic)

U–Pb and Lu–Hf isotope analyses of detrital zircons collected from metasedimentary rocks from the southern part of Kara Terrane (northern Taimyr and Severnaya Zemlya archipelago) provide vital information about the paleogeographic and tectonic evolution of the Russian High Arctic. The detrital zircon signatures of the seven dated samples are very similar, suggesting a common provenance for the clastic detritus. The majority of the dated grains belong to the late Neoproterozoic to Cambrian ages, which suggests the maximum depositional age of the enclosing sedimentary units to be Cambrian. The εHf(t) values indicate that juvenile magma mixed with evolved continental crust and the zircons crystallized within a continental magmatic arc setting. Our data strongly suggest that the main provenance for the studied clastics was located within the Timanian Orogen. A review of the available detrital zircon ages from late Neoproterozoic to Cambrian strata across the wider Arctic strongly suggests that Kara Terrane, Novaya Zemlya, Seward Peninsula (Arctic Alaska), Alexander Terrane, De Long Islands, and Scandinavian Caledonides all formed a single tectonic domain during the Cambrian age, with clastics predominantly sourced from the Timanian Orogen.

Geochemistry, geochronology and evolution of Paleoproterozoic granitoid gneisses in the Khondalite Belt, North China Craton

The studied granitoid gneisses from the Daqingshan complex in the Khondalite Belt of the North China Craton consist of tonalitic, trondhjemitic, and granodioritic (TTG) gneisses, as well as monzogranitic and granitic (MG) gneisses. The respective melts forming these granitoid gneisses are geochemically similar. The granitoid gneisses are generally enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, Nd, La, Ce and depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. Chondrite-normalized rare earth elements (REE) patterns indicate that both TTGs and MG gneisses illustrate various fractionation with (La/Yb)N ratios of 7.43–58.29 and 3.59–87.82, respectively. However, most TTG gneisses show positive Eu anomalies, and relatively negative Th and positive Sr anomalies, while MG gneisses have opposite characteristics. These geochemical characteristics suggest that TTG gneisses were derived from a magma source at higher pressures than that to have formed MG gneisses. Some TTG gneisses yield high Sr/Y (36–139) and (La/Yb)N ratios, indicating a subduction-related affinity. The whole series of granitoid gneisses from the Daqingshan complex belong to fractionated calc-alkaline I-type granitoids, suggesting their igneous protoliths akin to island arcs and continental magmatic arcs. LA-ICP-MS U–Pb dating of zircons from four representative granitoid samples indicates that tonalite and granodiorite exhibit weighted mean ages of 2416 ± 12 Ma and 2348 ± 10 Ma, respectively, whereas trondhjemite provides two groups of zircon ages yielding weighted mean ages of 2437 ± 15 Ma and 1876 ± 3 Ma, respectively. The studied granitic gneiss reveals an upper intercept of 1947 ± 45 Ma. Integration of cathodoluminescence images, Th/U ratio, and REE patterns of zircons indicates that all zircons of granitoid gneisses have a magmatic origin. Therefore, three major episodes of granitic magmatism at ca. 2.35–2.42, 1.95, and 1.88 Ga can be inferred. The ages of 1.95 and 1.88 Ga are coeval with widespread metamorphic events recorded by mafic and pelitic granulites in the Khondalite Belt. The Khondalite Belt could have undergone long-term arc-continent accretion along the southern margin of the Yinshan Block during early to late Paleoproterozoic time, and subsequent continental collision between the Yinshan and Ordos Blocks occurred at ca 1.95–1.88 Ga.

Zircon U-Pb and Lu-Hf record from high-grade complexes within the Mantiqueira Complex: First evidence of juvenile crustal input at 2.4–2.2 Ga and implications for the Palaeoproterozoic evolution of the São Francisco Craton

The Mantiqueira Complex together with the Mineiro Belt and Juiz de Fora Complex, characterize a long-lived system of oceanic and continental magmatic arcs. The proposed accretion of these terranes in the Rhyacian resulted in the closure of the Minas Basin, a fundamental turning point for the geology of this area. Understanding this event is critical for determining the evolution of the South American continent. Despite its importance, there is a paucity of zircon U-Pb and Lu-Hf data available for the Mantiqueira Complex and no data from two complexes within the later.U-Pb zircon data were obtained from interpreted metasediments of those two complexes, the granulite facies Acaiaca Complex and Pedra Dourada Granulite. Four targeted samples yield cores with ages between 3.5 and 2.2 Ga and a large proportion of metamorphic grains and rims at ca. 2.1–2.0 Ga. The similarity between the detrital zircon signatures of the Acaiaca Complex and Pedra Dourada Granulite, and the Archaean inheritance of the Mantiqueira Complex, indicates that these three units are related. Potentially the Acaiaca Complex and Pedra Dourada Granulite either sourced the Mantiqueira Complex or the Mantiqueira intrusive suites reworked the protoliths of the Acaiaca Complex and Pedra Dourada Granulite.Hf-in-zircon data indicate two discrete sources for both the Acaiaca Complex and Pedra Dourada Granulite. An Archaean source (model ages of 3.2–2.6 Ga), which is consistent with the suggested origin of the Mantiqueira Complex as recycled Archaean crust sourced from the São Francisco Craton and reworked during the ca. 2.1 Ga Transamazonian orogeny. A second more juvenile source (model ages of 2.4–2.2 Ga) recorded exclusively within the more mafic samples is identical in age and Hf to that found in the Juiz de Fora Complex. This could indicate a close link between the Mantiqueira and Juiz de Fora Complexes in the period 2.2 to 2.0 Ga. Alternatively, these juvenile zircons may represent an as yet unidentified juvenile portion of the Mantiqueira arc.The results of this study are consistent with the prevailing theories for formation of the Mantiqueira Complex as a continental arc with cumulative proportion curves indicating an active-margin type depositional setting.

Chasing the mantle: Deciphering cryptic mantle signals through Earth's thickest continental magmatic arc

Understanding the magmatic processes occurring in the upper mantle and lowermost crust above subduction zones is paramount if we hope to understand the primary mechanisms driving arc magmatism. In the Central Volcanic Zone of the Andes (CVZ), extreme crustal thicknesses and the prominence of silicic magmatism in the Neogene have prevented a clear understanding of the primitive magma input into what is arguably the Earth's archetypal active continental margin. However, we propose that phase chemistry and equilibria from mafic mineral assemblages in basaltic andesite enclaves from the Purico-Chascon Volcanic Complex reveal rare constraints on the character of primitive mantle-derived magmas feeding the CVZ arc. We interpret olivine, Cr-spinel, and olivine-hosted melt inclusion compositions from these mafic assemblages to record the partial melting of wet, oxidized mantle peridotite and the production of high-Mg basalt in the upper mantle. In turn, clinopyroxene and spinel-free olivine are interpreted to reflect magma modification in the lower crust and the adiabatic ascent of differentiated basaltic magma up to ∼20 km. Interestingly, olivine-hosted Cr-spinel compositions suggest that magmas in the Purico-Chascon system ascend from the upper mantle to surface with no significant change in Fe-oxidation state despite obvious differentiation and crustal interactions. Finally, comparison of high Mg-basalt in melt inclusions from Purico-Chascon (CVZ arc) and CVZ back-arc lavas show that the magmas may share a common peridotite source. However, the peridotite beneath the arc appears to be more hydrous and oxidized than that beneath the back-arc, confirming that back-arc lava compositions may not adequately represent mafic magma input into the arc. We posit that the relationships between H2O-content, oxidation conditions, and mafic phase equilibria discussed in this study are directly linked to subduction zone geometry, and thus may be applicable to arcs globally.

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

AbstractThe Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular terrane to the south and the Farewell and other pericratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard pericratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the Alexander-Wrangellia-Peninsular terrane and mid- to Late Cretaceous arc-related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at ca. 81 Ma. They were deformed together ca. 80 Ma by southeast-verging fold-and-thrust–style deformation interpreted to represent final accretion of the Alexander-Wrangellia-Peninsular terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the back-arc flank of an active, south-facing island-arc system (Alexander-Wrangellia-Peninsular terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench–Nankai Trough on the east side of central Japan.

Coeval high Ba-Sr arc-related and OIB Neoproterozoic rocks linking pre-collisional magmatism of the Ribeira and Araçuaí orogenic belts, SE-Brazil

Two arc systems have been described within the Neoproterozoic Ribeira Belt: the inner continental magmatic arc and the outer intra-ocean magmatic arc. New geochemical and U-Pb and Lu-Hf (LA-ICPMS) data from zircons of the Marceleza-Leopoldina stocks, belonging to the inner magmatic arc system, in the boundary region between the Ribeira and Araçuaí orogenic belts are presented. The intermediate to felsic magmatic rocks have high Ba, Sr, K/Rb, LILE and LREE contents. Low Rb, Th, U, Nb, and very low Y and HREE contents point to their generation in a supra-subduction tectonic scenario, from melting of the asthenospheric wedge above a subducting oceanic plate. Zircon U-Pb (LA-ICPMS) data indicates crystallization ages between ca. 650 and 600 Ma, with high-grade metamorphic overprint at ca. 582 Ma. Tonian inherited zircons were found in the older rocks, suggesting that the development of the continental magmatic arc might have begun earlier. Associated basic (gabbros and monzogabbros) to intermediate (diorites and monzodiorites) rocks are geochemically heterogeneous. Some represent less evolved calc-alkaline basaltic magmas, while relatively more enriched varieties are intraplate basalts with OIB components. Together with lithogeochemical data, Lu-Hf signatures showing TDM model ages between ca. 1.7 to 1.3 Ga, with initial ɛHf values ranging from slightly negative values (−0.9 to −3.75) to strongly negative (up to −10.4) indicate a magmatic arc developed on continental margin environment (i.e., continental-type arc). The studied rocks represent the Rio Doce arc in the studied region and are correlated with other arc segments located from the north, in the Araçuaí Orogen, to the southernmost Ribeira Orogen, defining a c. 1000 km long continental-type magmatic arc. This inner arc system first collided against the São Francisco craton during the Neoproterozoic Brasiliano orogeny around 580 Ma. The geochemical diversity of coeval intermediate to felsic rocks and the variability of associated basic rocks are suggestive of a complex evolution of the subduction zone system. Possible mechanisms for the enrichment in incompatible elements of magmatic products may have involved slab roll-back and subduction of fore-arc rocks, and the influx of ascending asthenosphere associated with slab windows (slab break off, ridge subduction) prior to collision.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Contemporaneous Paleogene arc-magmatism within continental and accreted oceanic arc complexes in the northwestern Andes and Panama

The basement rocks of the northwestern Andes are composed of accreted, fault bound, slivers of the Cretaceous Caribbean Large Igneous Province (CLIP). Previous geologic mapping, geochronological and geochemical data show that Paleogene arc-like units make up part of the accreted CLIP in SW Colombia (Timbiquí Complex), NW Colombia (Santa Cecilila La Equis Complex), and at the boundary between Colombia and Panama (Acandí Region). Similar Paleogene igneous units have been reported in Ecuador (Silante and Macuchí arc), and Panama (Chagres-Bayano Arc). These arc-like rocks are commonly interpreted to have formed as part of an intra-oceanic arc before accretion to the continental margin. Alternative explanations suggest that the Paleogene igneous rocks in Ecuador were formed within a continental magmatic arc. We compare Paleogene igneous rocks in the San Blas Complex (Chagres-Bayano Arc and Acandí Region), Santa Cecilia la Equis Complex, and the SW Complex (Timbiquí Complex, Silante and Macuchí arcs) using geochemical (major and trace elements), geochronological (zircon U-Pb) and isotopic (zircon Hf, and whole rock Pb, Sr, Nd isotopes) data. Zircon U-Pb analyses yield ages of 46–42 Ma for the Acandí Region, 59–51 Ma for the Santa Cecilia La Equis Complex, and 44–42 Ma for the Timbiquí Complex. Major and trace element data show low-K to calc-alkaline compositions with negative Nb anomaly and LREE/HREE enrichment typically attributed to arc-like signatures, for the three complexes. Pb, Sr, and Nd whole rock and zircon Lu-Hf isotopic data for the Santa Cecilia La Equis and the San Blas complexes, are consistent with a depleted mantle source mixed with Pacific sediments. However, the presence of inherited Proterozoic zircons, elevated 207Pb/204Pb ratios in some evolved igneous rocks, and crystallization under low pressure conditions, suggest assimilation of continental-derived sediments. These data indicate that the Santa Cecilia La Equis and the San Blas complexes likely originated in an intra-oceanic island arc setting, in a close proximity to the South American margin before accretion to the Andean margin. In the case of the SW Complex, the absence of Eu anomaly, slightly depleted HREE, as well as increased Sr/Y values with fractionation, indicate that these magmas formed in a moderately mature arc within a mid-shallow crust level. Moreover, radiogenic Pb and Sr isotopic values for the less evolved igneous rocks of the SW Complex evolving to less radiogenic values suggest involvement of continental sediments in the parental magma source, and assimilation of the mafic basement crust. This indicates that the Paleogene magmatic rocks of the SW Complex originated in a continental arc-like setting within previously accreted slivers of the CLIP.

On the role of chemical weathering of continental arcs in long-term climate regulation: A case study of the Peninsular Ranges batholith, California (USA)

The long-term stability of atmospheric pCO2 is dominantly determined by the balance between the rate of CO2 input from magmatic/metamorphic degassing and the efficiency of CO2 uptake by silicate weathering via eventual precipitation as marine carbonate and by organic carbon burial. Silicate weathering is thought to represent a negative feedback to changes in atmospheric pCO2. The Late Cretaceous-Early Cenozoic was characterized by elevated atmospheric pCO2 and greenhouse climate, widely thought to be due to increased magmatic flux from mid-ocean ridges, flood basalts and continental arcs. Of interest here is the role of continental arcs in modulating long term atmospheric CO2 contents and climate. Continental arc magmatism is accompanied by rapid uplift and erosion due to magmatic/tectonic thickening of the crust, thus, the development of continental arcs might also enhance the global efficiency of silicate weathering, in turn increasing the efficiency of carbon sequestration. To assess the contribution of continental arcs to global carbon sinks, we conducted a case study in the Cretaceous Peninsular Ranges batholith (PRB) and associated forearc basin in southern California, USA, representing one segment of a Mesozoic circum-Pacific continental arc system. Arc magmatism occurred between 170-85 Ma, peaking at 100 Ma. Erosion occurred during magmatism and continued well into the early Eocene, with forearc sediments representing the products of this protracted unroofing of the arc. By calculating the depletion of Ca in forearc sediments relative to their arc protoliths, we estimate chemical weathering fluxes (in equivalent CO2) to be ∼106 mol km−2yr−1 near the end of magmatism, decreasing to ∼105 mol km−2yr−1 by the Early Eocene. Integrated over the entire magmatic and post-magmatic history of the batholith, the total amount of CO2 consumed through chemical weathering is comparable to that degassed. However, during magmatism, the regional degassing flux exceeds the regional weathering flux even though the efficiency of weathering is increased due to magmatic and tectonic uplift. After magmatism ends, continued erosion from remnant topography causes the arc to transition into a net regional sink, increasing the global efficiency of chemical weathering and amplifying cooling after magmatism ends. Understanding how atmospheric CO2 and climate vary over long timescales requires an understanding of how magmatic orogenies enhance degassing but also increase the strength of the global silicate weathering feedback.

Geochemical constraints on the origin and tectonic setting of Chargar intrusions in the Alborz orogenic belt, NW Iran

Chargar area is located in the southern border of Tarom subzone within the Alborz magmatic belt of NW Iran. Two types of intrusions, mainly present in the southern part of the area, have been identified. These bodies intruded into the Karaj Formation volcanic and volcaniclastic rocks. Type-I intrusions occur in the south and include two magmatic bodies: (a) Gabbro-pyroxene quartz monzodiorite-quartz monzodiorite and (b) quartz syenite. Type-II crops out in the west and has a gabbro – gabbro-diorite composition. Geochemically, Chargar intrusive rocks belong to the high-K calc-alkaline to shoshonitic series and are classified as I-type and metaluminous granitoids. These intrusive rocks show insignificant different distribution patterns of trace elements and REEs, but generally they are characterized by highly enriched large ion lithophile elements (e.g., K and Ba) and depleted Nb, similar to that of continental arc magmatism. Type-I intrusions and Type-II mafic intrusion were originated from mantle source with amphibole-bearing spinel peridotite composition. These intrusions formed in a continental arc to post-collisional tectonic setting.

A back-arc origin for the Neoarchean megacrystic anorthosite-bearing Bird River Sill and the associated greenstone belt, Bird River subprovince, Western Superior Province, Manitoba, Canada

This study presents extensive new field, petrographic, and whole-rock major and trace element and Sm–Nd and Pb isotope data for the Neoarchean megacrystic anorthosite-bearing Bird River Sill, spatially associated Neoarchean supracrustal rocks of the Bird River greenstone belt, and the Mesoarchean Maskwa Lake TTG Batholith I, Manitoba, Canada. Field observations indicate that the 2743 Ma Bird River Sill was emplaced into the Northern Lamprey Falls Formation in an oceanic setting and subsequently intruded by the ca. 2725 Ma Maskwa Lake Batholith II. The Northern Lamprey Falls Formation is in fault contact with the overlying Peterson Creek Formation, which in turn is unconformably overlain by the Bird River Formation. The peridotites of the Bird River Sill were subjected to intense alteration and ductile shearing, resulting in the redistribution of many major and trace elements and resetting of their Sm–Nd and U–Th–Pb isotopic systems. The gabbro, leucogabbro, and anorthosite units of the sill largely retain their primary igneous textures and minerals, REE and HFSE compositions, and Sm–Nd and Pb isotope signatures. These geochemical and petrographic data indicate that these units of the Bird River Sill crystallised from a hydrous, Ca- and Al-rich tholeiitic magma that fractionated from a hydrous primitive arc tholeiitic parental magma. The major and trace element geochemistry and initial eNd values of the Bird River Sill (+ 0.53 to + 4.72) and the Northern Lamprey Falls Formation (+ 1.40 to + 2.66) of the Bird River greenstone belt crystallised from parental magma batches that were derived by partial melting of variably-depleted mantle sources. High-magnesian andesites from the Bird River Formation formed by partial melting of subducting oceanic crust and/or slab sediments and subsequent interaction with sub-arc mantle peridotite, whereas the Peterson Creek Formation dacites and Maskwa Lake TTG Batholith I tonalites formed by partial melting of juvenile lower arc crust. The major and trace element geochemistry of the Bird River Sill and greenstone belt and the Maskwa Lake TTG Batholith I, and geological relationships and lithological associations point to a transition from continental arc to continental back-arc magmatism along the southern active margin of the North Caribou terrane at ca. 2743 Ma, resulting in the formation of the Maskwa Lake microcontinent that occurred prior to subsequent continental arc magmatism along the southern margin of this microcontinent. Combined field observations and geochemical data indicate that the Bird River Sill and Bird River greenstone belt are a dismembered Archean subduction-related ophiolite that marks a suture zone between the Winnipeg River subprovince and the Maskwa Lake microcontinent.

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Geology, mineralization, fluid inclusion, and stable isotope characteristics of the Sinyukhinskoe Cu-Au skarn deposit, Russian Altai, SW Siberia

The Sinyukhinskoe deposit (>100 t Au with average 9.2 g/t Au) is situated in the Altaid orogenic system and is part of the regional system of Devonian-Carboniferous Cu-Au to Au-Cu porphyry, skarn and other deposits. Productive plutonic suites were emplaced in a Devonian subduction-related continental magmatic arc setting, with its further evolution toward a late- and post-collisional environment in the late Devonian-Carboniferous. The deposit is related to a magnetite-series, I-type multiphase pluton, with the evolution of the intrusive phases from early gabbroic through diorite-granodiorite to the late granitic phases. The igneous rocks belong to medium-K calc-alkaline series or are transitional from medium- to high-K calc-alkaline varieties. Prior to granitic phases, this evolution was interrupted by the intrusion of intramineral high-K mafic dikes exhibiting a shoshonitic affinity.The deposit is characterized by the development of abundant andradite-rich garnet and wollastonite in prograde and retrograde skarns, locally together with magnetite and bornite. The subsequent propylitic (epidote-chlorite-amphibole-albite-calcite-quartz, locally with clinozoisite and prehnite) alteration assemblages bear more intense Cu mineralization (chalcopyrite, bornite, locally tennantite) and high fineness (~940–910‰) native gold. Later phyllic (quartz-sericite to quartz-sericite-chlorite) alteration assemblages comprise the most intense intermediate-fineness (910–880‰) native gold and Cu (bornite, chalcocite) mineralization that is followed by low-fineness (820–760‰) native gold and telluride minerals.The retrograde skarn was formed from a high-salinity (22–23 wt% NaCl-eq.), hot (450 ± 50 °C), high-pressure (2.5 ± 0.5 kbar) homogenous magmatic-hydrothermal fluid, under its possible direct separation from crystallizing magma. After intrusion of intramineral mafic dikes, propylitic alteration assemblages were formed from a moderate salinity (15–18 wt% NaCl-eq.) homogenous aqueous fluid at the temperatures of 380–400 °C. This fluid could be supplied by crystallizing magma, with its further mixing with heated meteoric waters. Fluid inclusions in phyllic alteration minerals indicate a new influx of homogenous, high-salinity (18 wt% NaCl, 22 wt% KCl, 10 wt% CaCl2), high-pressure (1.3 ± 0.1 kbar), aqueous-chloride fluid at lower (~280 °C) temperatures. Elevated Bi, Hg, Te contents, as well as δ34S values close to meteorite standard (0 ± 1.5‰) suggest at least partial fluid sourcing from a mantle-derived magma at the phyllic alteration stage.

Zircon U–Pb geochronology, mineral and whole‐rock geochemistry of the Khardung volcanics, Ladakh Himalaya, India: Implications for Late Cretaceous to Palaeogene continental arc magmatism

In this study, we present new mineral and whole‐rock geochemical data with zircon U–Pb ages of the Khardung volcanics (KV) from the western Himalaya and discuss their tectono‐magmatic evolution. These volcanics are sandwiched between the Ladakh batholith and Karakoram batholith and classified as intermediate volcanics (basaltic andesite‐andesite) and felsic volcanics (dacite‐rhyolite). The intermediate volcanics are marked by low SiO2 (52.80–61.31 wt.%), enriched LREEs, and depleted HFSEs (Nb, Ti, Zr), whereas more evolved felsic volcanics exhibit quartz, K‐feldspar, and plagioclase as dominant mineral phases and felsic compositions are characterized by high SiO2 (64.52–79.19 wt.%) content with pronounced negative Eu anomalies, enriched LREEs, and depleted HREEs and HFSEs (Nb, Ti). New zircon U–Pb ages of intermediate volcanics (andesite) yield 69.71 Ma, whereas felsic volcanics (rhyolites) range between 62.49 and 66.55 Ma, indicating that the Khardung magmatism overlaps with the last phase of the Ladakh batholith magmatism. Geochemical characteristics indicate that the KV were generated from a same parental magma source through fractional crystallization along with crustal assimilation from an older crust, and they show genetic affinity with the adjacent Ladakh batholith. Therefore, the KV and Ladakh batholith could be considered as a product of the mature stage arc magmatism generated during subduction of the Neo‐Tethyan oceanic crust prior to the main collision between the Indian and Eurasian continents.

Resolving tectonic settings of ancient magmatic suites using structural, geochemical and isotopic constraints: the example of the St Peter Suite, southern Australia

Reconstructing ancient tectonic environments is inherently difficult and requires the integration of many datasets. Here we attempt to reconstruct the tectonic setting of a Proterozoic magmatic rock suite from the Archean to Mesoproterozoic Gawler Craton, South Australia. We evaluate previous models of subduction or non-subduction tectonics for the St Peter Suite through field observations coupled with geochemical and isotopic data. Field observations indicate the presence of mafic magma chambers, felsic plutons and zones of mixing represented by multiphase plutons and zones of magma transfer. Syn-magmatic and solid-state deformation fabrics suggest magmatism was associated with a compressional tectonic regime, at least during some stages of magmatism. Zircon U–Pb geochronology suggests magmatism occurred between ca 1633 and ca 1608 Ma, an interval of ca 25 Myr. Mafic rocks are tholeiitic and enrichment in high field strength and light rare earth elements supports a modified mantle source region. Mafic rocks have εNd1620 Ma values around 0, and zircon εHf(t) values around 4, supporting minor crustal contamination. Felsic plutons have a geochemical signature similar to average continental crust, εNd1620 Ma between –3.7 and 0.4, and zircon Lu–Hf isotopic compositions that yield to εHf(t) values between –1.8 and 7.7, suggesting they were derived from fractionation of enriched tholeiitic parent magmas. A model for the formation of the St Peter Suite that is most consistent with the available data is one that places the magmatism in a broadly continental magmatic arc setting, albeit one in which little or no Archean crust was involved. Potentially this magmatic arc could have been built on a thinned, or hyper-extended continental margin. The integration of structural, geochemical and isotopic constraints has provided a more holistic view of the St Peter Suite petrogenesis than previously available.HighlightsEvaluation of models of subduction vs non-subduction tectonics for formation of Paleoproterozoic magmatic suiteEvidence for syn-magmatic compression and mafic–felsic magma minglingIsotopically juvenile (Nd--Sr--Hf) with mantle-like zircon δ18O valuesContinental arc setting proposed, possibly on hyper-extended continental margin

A survey of Sierra Nevada magmatism using Great Valley detrital zircon trace-element geochemistry: View from the forearc

AbstractThe well-characterized Sierra Nevada magmatic arc offers an unparalleled opportunity to improve our understanding of continental arc magmatism, but present bedrock exposure provides an incomplete record that is dominated by Cretaceous plutons, making it challenging to decipher details of older magmatism and the dynamic interplay between plutonism and volcanism. Moreover, the forearc detrital record includes abundant zircon formed during apparent magmatic lulls, suggesting that understanding the long-term history of arc magmatism requires integrating plutonic, volcanic, and detrital records. We present trace-element geochemistry of detrital zircon grains from the Great Valley forearc basin to survey Sierra Nevadan arc magmatism through Mesozoic time. We analyzed 257 previously dated detrital zircon grains from seven sandstone samples of volcanogenic, arkosic, and mixed compositions deposited ca. 145–80 Ma along the length of the forearc basin. Detrital zircon trace-element geochemistry is largely consistent with continental arc derivation and shows similar geochemical ranges between samples, regardless of location along strike of the forearc basin, depositional age, or sandstone composition. Comparison of zircon trace-element data from the forearc, arc, and retroarc regions revealed geochemical asymmetry across the arc that was persistent through time and demonstrated that forearc and retroarc basins sampled different parts of the arc and therefore recorded different magmatic histories. In addition, we identified a minor group of Jurassic detrital zircon grains with oceanic geochemical signatures that may have provenance in the Coast Range ophiolite. Taken together, these results suggest that the forearc detrital zircon data set reveals information different from that gleaned from the arc itself and that zircon compositions can help to identify and differentiate geochemically distinct parts of continental arc systems. Our results highlight the importance of integrating multiple proxies to fully document arc magmatism, demonstrating that detrital zircon geochemical data can enhance understanding of a well-characterized arc, and these data may prove an effective means by which to survey an arc that is inaccessible and therefore poorly characterized.