Primitive Island Arc Research Articles

Tectonomagmatic setting and provenance of the Santa Marta Schists, northern Colombia: Insights on the growth and approach of Cretaceous Caribbean oceanic terranes to the South American continent

Metamorphosed volcano-sedimentary rocks accreted to the northern South American continental margin are major vestiges of the Caribbean oceanic plate evolution and its interactions with the continent. Selected whole rock geochemistry, Nd–Sr isotopes and detrital zircon geochronology were obtained in metabasic and metasedimentary rocks from the Santa Marta and San Lorenzo Schists in northernmost Colombia. Trace element patterns are characterized by primitive island arc and MORB signatures. Similarly initial 87Sr/86Sr-εNd isotopic relations correlate with oceanic arcs and MORB reservoirs, suggesting that the protoliths were formed within a back-arc setting or at the transition between the inta-oceanic arc and the Caribbean oceanic crust. Trace element trends from associated metasedimentary rocks show that the provenance was controlled by a volcanic arc and a sialic continental domain, whereas detrital U/Pb zircons from the Santa Marta Schists and adjacent southeastern metamorphic units show Late Cretaceous and older Mesozoic, Late Paleozoic and Mesoproterozoic sources. Comparison with continental inland basins suggests that this arc-basin is allocthonous to its current position, and was still active by ca. 82 Ma. The geological features are comparable to other arc remnants found in northeastern Colombia and the Netherland Antilles. The geochemical and U/Pb detrital signatures from the metasedimentary rocks suggest that this tectonic domain was already in proximity to the continental margin, in a configuration similar to the modern Antilles or the Kermadec arc in the Pacific. The older continental detritus were derived from the ongoing Andean uplift feeding the intra-oceanic tectonic environment. Cross-cutting relations with granitoids and metamorphic ages suggest that metamorphism was completed by ca. 65 Ma.

Geochemical characteristics of the Río Verde Complex, Central Hispaniola: Implications for the paleotectonic reconstruction of the Lower Cretaceous Caribbean island-arc

New geochronological, trace element and Sr–Nd isotope data for metabasalts, dolerites and amphibolites from the Río Verde Complex, Central Hispaniola, are integrated with existing geochemical data for mafic volcanic rocks and metamorphic derivatives from the Los Ranchos, Amina and Maimón Formations, giving new insights into magma petrogenesis and paleotectonic reconstruction of the Lower Cretaceous Caribbean island-arc–back arc system. U–Pb and 40Ar/ 39Ar age data show that the Río Verde Complex protoliths were in part coeval with volcanic rocks of the Los Ranchos Formation (Upper Aptian to Lower Albian). The geochemical data establish the existence of gradients in trace element parameters (Nb/Yb, Th/Yb, Zr/Yb, Zr/Ba, and normalized Ti, Sm, Y and Yb abundances) and Nd isotope compositions from throughout Hispaniola, which reflect differences in the degree of mantle wedge depletion and contributions from the subducting slab. The Río Verde Complex mafic rocks and some mafic sills and dykes intruding in the Loma Caribe Peridotite, have a transitional IAT to N-MORB geochemistry and a weak subduction-related signature, and are interpreted to form in a rifted arc or evolving back-arc basin setting. The Los Ranchos, Amina and Maimón Formations volcanic rocks have arc-like characteristics and represent magmatism in the volcanic front. Trace element and Nd isotope modeling reproduce observed data trends from arc to back-arc and suggest that the variations in several geochemical parameters observed in a SW direction across the Caribbean subduction system can be explained from the progressively lower subduction flux into a progressively less depleted mantle source. The low Nb contents and high ( ε Nd) i values in both arc and back-arc mafic rocks imply, however, the absence of a significant Lower Cretaceous plume enriched component. In order to explain these observations, a model of proto-Caribbean oceanic lithosphere subducting to the SW at least in the 120–110 Ma interval, is proposed to cause the observed magmatic variations in the Lower Cretaceous Caribbean island-arc–back-arc system. In this context, arc rifting and initial sea-floor spreading to form the Río Verde Complex protoliths occurred in the back-arc setting of this primitive island-arc, built on the NE edge of the Caribbean plate.

Accretion of juvenile crust at the Early Palaeozoic Antarctic margin of Gondwana: geochemical and geochronological evidence from granulite xenoliths

AbstractGeodynamic models for the Antarctic sector of the active Early Palaeozoic Palaeo‐Pacific margin of Gondwana are based on the nature and age of the deep crust of the Robertson Bay terrane, the outermost lithotectonic unit of the margin. As this crustal block is covered with thick turbidite deposits, the only way to probe the deep crust is through the analysis of granulite xenoliths from Cenozoic scoria cones. Low‐K felsic xenoliths yield the oldest (Middle Cambrian) laser‐probe U–Pb ages on zircon areas with igneous growth zoning. This finding, along with the positive whole‐rock εNd(500Ma), suggests that these felsic rocks derived from a juvenile magma formed during the Early Palaeozoic Ross orogenic cycle. Mafic xenoliths have geochemical‐isotopic compositions similar to those of modern primitive island arcs, suggesting the involvement of subducted oceanic crust in their magma genesis and accretion of juvenile crust at the Antarctic margin of Gondwana.

Is the Cretaceous primitive island arc series in the circum-Caribbean region geochemically analogous to the modern island arc tholeiite series?

Abstract The Early Cretaceous island arc lavas in the Caribbean region are frequently assigned to the primitive island arc (PIA) series and not to the island arc tholeiite (IAT) series. However, this review demonstrates that the Caribbean PIA rocks have immobile trace element abundances, trace element ratios and Nd–Hf isotope systematics which are indistinguishable from modern IAT lavas. Thus, it is proposed that the term PIA series be discarded and that the Early Cretaceous island arc rocks in the Caribbean be classified as IAT rocks.

Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust

We report the results of a geochemical study of the Jijal and Sarangar complexes, which constitute the lower crust of the Mesozoic Kohistan paleo-island arc (Northern Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite with minor lenses of garnet hornblendite and granite, grading up-section to hornblende gabbronorite. The Sarangar complex is composed of metagabbro. The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, light rare earth element (LREE)-enriched REE patterns similar to those of island arc basalts. Together with the Jijal garnet granulite, they define negative covariations of LaN, YbN and (La/Sm)N with Eu* [Eu* 1⁄4 2 · EuN/(SmN þ GdN), where N indicates chondrite normalized], and positive covariations of (Yb/Gd)N with Eu*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of the garnet granulites as plagioclase–garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration-melting of hornblende gabbronorite, provided that the amount of segregated or restitic garnet was low (<5 wt %). Field, petrographic, geochemical and experimental evidence is more consistent with formation of the Jijal garnet granulite by dehydration-melting of Jijal hornblende gabbronorite. Similarly, the Jijal garnet-bearing hornblendite lenses were probably generated by coeval dehydration-melting of hornblendites. Melting models and geochronological data point to intrusive leucogranites in the overlying metaplutonic complex as the melts generated by dehydration-melting of the plutonic protoliths of the Jijal garnetbearing restites. Consistent with the metamorphic evolution of the Kohistan lower arc crust, dehydration-melting occurred at the mature JOURNAL OF PETROLOGY VOLUME 47 NUMBER 10 PAGES 1873–1914 2006 doi:10.1093/petrology/egl030

Pb, Sr, and Nd isotopic and chemical evidence for a primitive island arc emplacement of the El Arco porphyry copper deposit (Baja California, Mexico)

The El Arco porphyry copper deposit is located in central Baja California and is a resource containing >600 Mt of ore with ∼0.6% copper. It was emplaced within a relatively primitive Jurassic island arc and it was subsequently metamorphosed and intruded by the Cretaceous Peninsular Ranges batholith. The porphyritic stock intrusion and ore formation at El Arco has recently been dated at ∼165 Ma [Valencia et al. GEOS 24:189 (2004)]. This age is much older than Aptian-Albian K–Ar ages previously reported from El Arco [Barthelmy, Geology of El Arco-Calmalli area, Baja California, Mexico. MSc Thesis, San Diego State University, CA (1975); Baja California Geology. San Diego State University, CA, pp 127–138 (1979)]. The copper mineralization at El Arco is concentrated in a core of potassic alteration in a dioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas. Mafic dikes that intruded the deposit are not mineralized, but they are affected by post-ore low-grade metamorphism. The dikes are compositionally the most primitive rocks, while host rock andesites and the porphyry stock display typical volcanic arc characteristics. The Pb isotope data from sulfides, feldspars, quartz, and whole-rock samples indicate that: (1) the copper-bearing porphyry stock and the surrounding andesites evolved from a similar source with an average μ-value of 9.43; (2) no external Pb was added during mineralization; (3) some Pb isotope compositions were slightly disturbed by a later metamorphic event. Strontium and Nd isotopes show that the magmas evolved from a depleted mantle reservoir with no involvement of older continental crust. Our data favor a model for the formation of the El Arco deposit linked to a Triassic to Jurassic intra-oceanic arc system, cropping out at the western margin of central Baja California in the Cedros-Vizcaino region. The intra-oceanic arc together with the El Arco deposit was accreted to the active continental margin of North America and metamorphosed during the Early Cretaceous. This model is in disagreement to earlier models that favor the El Arco deposit formation being linked to the Cretaceous continental margin.

Age of the Los Ranchos Formation, Dominican Republic: Timing and tectonic setting of primitive island arc volcanism in the Caribbean region

This study reports U-Pb ages for zircons from the Los Ranchos Formation, which is part of the primitive island arc sequence, the oldest volcanic rocks in the Greater Antilles. Zircons were analyzed from three samples: quartz porphyry (quartz keratophyre) from the lower part of the formation (Quita Sueno Member), fragmental quartz porphyry from the upper part of the formation (Pueblo Viejo Member), and the Cotui quartz diorite stock that intrudes the lower part of the formation. The lower part of the Los Ranchos Formation, represented by the Quita Sueno sample, formed at either 113.9 ± 0.8 or 118.6 ± 0.5 Ma depending on interpretation of the data, whereas the upper part, represented by the Pueblo Viejo sample, formed ca. 110.9 ± 0.8 Ma. The Cotui stock (RD-73-601) was emplaced ca. 111.8 ± 0.6 Ma or 112.9 ± 0.9 Ma depending on whether eight grains reflect subtle inheritance. These results show that the Los Ranchos Formation was emplaced during the Aptian-Albian transition, that the Cotui stock could have supplied magma to the volcanic sequence and fluids to the Pueblo Viejo gold-silver deposit in the upper part of the formation, and that carbonaceous sediments in the upper part of the formation formed at the same time as ocean anoxic event 1b (Paquier). Caribbean plate tectonic models involving invasion of anomalous Pacific crust into the Caribbean region appear to agree best with this age because they provide a mechanism for the change from primitive island arc (PIA) to calcalkaline magmatism and account for the restricted ocean circulation necessary to generate regional anoxic conditions.

Serpentinitic Mélanges Associated with HP and UHP Rocks in Central Asia

Serpentinitic mélanges containing eclogites, blueschists, and jadeitic rocks have been studied within the following foldbelts: Borus (West Sayan), Chagan-Uzun (Gorny Altai), Chara (East Kazakhstan), Kokchetav (North Kazakhstan), and Maksyutov (Southern Urals). The West Sayan (Borus belt) and Gorny Altai (Chagan-Uzun belt) are Caledonian foldbelts formed after collisions of paleo-seamounts with primitive island arcs, which resulted in the exhumation of eclogites and jadeitic rocks. The Chara belt of Hercynian age was formed during the Late Carboniferous-Permian collision of the Siberian and Kazakhstan continents and represents a strike-slip zone comprising serpentinitic mélanges and HP units of different ages. In Caledonian time, the mélanges represent an accretionary wedge at the margin of the Kazakhstan continent. The Kokchetav and Maksyutov foldbelts of, respectively, Caledonian and Hercynian ages resulted from collision of microcontinents with island arcs and formation of sheeted complexes incorporating large lenses of UHP-HP rocks and less serpentinitic mélanges. Two types of serpentinitic mélange occur in all suture zones formed as a result of collision of island arcs with seamounts or other island arcs. Type I mélange contains HP rocks in antigorite or olivine-talc-antigorite schists associated with high-temperature mantle peridotites, which can be regarded as part of the hanging wall of a paleosubduction zone. Type II mélange is a part of an obducted oceanic crust and contains blocks and inclusions of LP and LT metamorphic rocks associated with island arc and/or seamount terranes. The suture zones formed by continent/island arc or microcontinent collision (the Kokchetav megamélange and the lower unit of the Maksyutov complex) comprise no large sheets of serpentinitic mélange, but only subordinate lenses of mélanges with HP rocks (the Maksyutov lower unit) or small and sporadic ultramafic lenses inside a tectonic mélange with a metasedimentary matrix.

Fragments of oceanic islands in accretion–collision areas of Gorny Altai and Salair, southern Siberia, Russia: early stages of continental crustal growth of the Siberian continent in Vendian–Early Cambrian time

The Altai-Salair area in southern Siberia is a Caledonian folded area containing fragments of Vendian–Early Cambrian island arcs. In the Vendian–Early Cambrian, an extended system of island arcs existed near the Paleo-Asian Ocean/Siberian continent boundary and was located in an open ocean realm. In the present-day structural pattern of southern Siberia, the fragments of Vendian–Early Cambrian ophiolites, island arcs and paleo-oceanic islands occur in the accretion–collision zones. We recognized that the accretion–collision zones were mainly composed of the rock units, which were formed within an island-arc system or were incorporated in it during the subduction of the Paleo-Asian Ocean under the island arc or the Siberian continent. This system consists of accretionary wedge, fore-arc basin, primitive island arc and normal island arc. The accretionary wedges contain the oceanic island fragments which consist of OIB basalts and siliceous—carbonate cover including top and slope facies sediments. Oceanic islands submerged into the subduction zone and, later were incorporated into an accretionary wedge. Collision of oceanic islands and island arcs in subduction zones resulted in reverse currents in the accretionary wedge and exhumation of high-pressure rocks. Our studies of the Gorny Altai and Salair accretionary wedges showed that the remnants of oceanic crust are mainly oceanic islands and ophiolites. Therefore, it is important to recognize paleo-islands in folded areas. The study of paleo- islands is important for understanding the evolution of accretionary wedges and exhumation of subducted high-pressure rocks.

The basement of the Deseado Massif at Bahı́a Laura, Patagonia, Argentina: a proposal for its evolution

The Rı́o Deseado Complex, the basement of the eastern Deseado Massif geological province of Argentina, is composed of five dispersed, small outcrops of upper Proterozoic–lower Paleozoic metamorphic and igneous rocks. We propose a possible evolution for these rocks on the basis of a detailed study of the easternmost outcrop (Bahı́a Laura). The Bahı́a Laura metamorphic rocks are metasedimentary and igneous-derived rocks formed prior to the Pampean metamorphic event. Metasedimentary schists, garnet-rich and micaceous gneisses, and the migmatite's paleosome are interpreted as marine sediments (pelites, sandstones, and limestones). The igneous-derived rocks are metaluminous and peraluminous granodiorites (hornblendic and granodioritic gneisses) and tholeiitic basalts (amphibolites) that were produced in a magmatic arc and a primitive island arc, respectively. These rocks underwent regional metamorphism during the Pampean cycle (Neoproterozoic–Paleozoic boundary), then a plutonic intrusion (Bahı́a Laura peraluminous monzogranite) and several granitoid injections,both of which are correlated with the Famatinian (Ordovician–Silurian) magmatic arc proposed for this region. Finally, all rocks were slightly deformed by a post-Famatinian tectonic event that may have produced the basement exhumation. According to the proposed evolution of the Rı́o Deseado Complex in Bahı́a Laura, the basement of the Deseado Massif could be part of a Late Proterozoic-Mid Paleozoic orogenic belt.

Neoproterozoic to Early Ordovician Evolution of the Paleo-Asian Ocean: Implications to the Break-up of Rodinia

The paper reviews and integrates the recent geological and geochronological data, which allow us to recognize three stages of the evolution of the Paleo-Asian Ocean. The opening of the Paleo-Asian Ocean at 970-850 Ma is dated by the Nersin Complex in the Aldan shield, plagiogranites of the Sunuekit massif, enderbites of the Sludinsk Lake area, and passive margin sediments of the Patoma or Baikal series. The initial subduction (850-700 Ma) is marked by volcanic rocks, trondjemite and gabbro of the Sarkhoy island arc series. Collisions of microcontinents with Siberia at 660 to 620 Ma are evidenced by the exhumation of Muya eclogites (650 Ma), formation of migmatites and amphibolites of the Njurundukan belt (635 and 590 Ma), metamorphic units of the Near-Yenisei belt (640-600 Ma), and orogenic molasse (640-620 Ma). The Paleo-Asian Ocean maximally opened at 620-550 Ma, because at that time a long island arc composed of boninite volcanic rocks was formed. Primitive island arcs of that age have been reconstructed in Kazakhstan, Gorny Altai, West and East Sayan, and North Mongolia. HP and UHP rocks formed in two stages at 550-520 and 520-490 Ma. At 550-490 Ma oceanic islands and Gondwana-derived microcontinents (Kokchetav, Tuva-Mongolian, Central Mongolian and others) collided with the Cambrian-early Ordovician island arc of the Siberian continent. As a result, the island-arc system was extensively modified. Collision occurred twice at 550-520 and 520-490 Ma during which many HP and UHP rocks formed. At that time, the new oceans - the Junggar, Kazakhstan and Uralian - with an Ordovician island arc were formed.

Origin and emplacement of the Late Cretaceous Baer–Bassit ophiolite and its metamorphic sole in NW Syria

The Baer–Bassit ophiolite, of inferred Late Cretaceous age, was emplaced from the south Tethys ocean onto the leading edge of the Arabian continental margin in latest Cretaceous (Maastrichtian) time. Dismembered sequences in different thrust sheets can be correlated to produce a complete ophiolite sequence, with a metamorphic sole at the base, overlain, in turn, by upper mantle tectonite, rare cumulates, massive and layered gabbros, localized high-level plagiogranites, sheeted dykes, basic extrusives and minor Fe–Mn sediments (umbers). The restored ophiolite sequence is similar to that of the more intact Troodos and Hatay ophiolites, but dissimilar to Oman. The Baer–Bassit extrusives are magnesian and strongly depleted, comparable to primitive island arc tholeiites and some boninitic lavas, which favours a subduction-related origin. A well-developed metamorphic sole is divisible into high-grade (amphibolite facies) assemblages in the north and lower-grade (greenschist facies) assemblages in the centre of the region, but no complete inverted metamorphic gradient is preserved in any one local area. The protoliths of both the high-grade (amphibolites) and lower-grade rocks (greenschists) are interpreted as alkali basalts and pelagic sediments, including chert, similar to the volcanic rocks in an underlying unmetamorphosed melange. The metamorphic fabrics progressively evolved from ductile to brittle during tectonic transport towards the southeast. Most of the metamorphic fabrics evolved during intense shearing, but some of the alkaline metabasites apparently were metamorphosed under relatively static strain conditions. The apparent absence of preserved inverted metamorphic gradients in Baer–Bassit may reflect a complex deformation history during emplacement. Alternatively, differing P– T conditions may be recorded at different depths at the base of the over-riding mantle wedge. During emplacement, the front of the ophiolite was tectonically imbricated and overthrust by the main ultramafic slab (Bassit massif). The thickest massif (Baer) is underlain by a high-grade metamorphic sole and was internally shortened but without thrust duplication of the entire ophiolite sequence. Following covering by Late Maastrichtian–Palaeogene marine calcareous sediments, the area was subjected to mid-Tertiary regional folding. This was followed by Neogene dominantly left-lateral, strike-slip deformation along the African–Eurasian plate boundary, extending from south of Cyprus to the Dead Sea transform fault. As a result, the originally emplaced thrust sheets were dissected into three main composite units (Baer, Bassit and the Southeastern units), separated by strongly faulted and sheared ophiolitic blocks and unmetamorphosed volcanic–sedimentary melange.

A Vendian-Cambrian Island Arc System of the Siberian Continent in Gorny Altai (Russia, Central Asia)

An extended Vendian-Cambrian island-arc system similar to the Izu-Bonin-Mariana type is described in the Gorny Altai terrane at the margin of the Siberian continent. Three different tectonic stages in the terrane are recognized. (1) A set of ensimatic active margins including subducted oceanic crust of the Paleo-Asian ocean, the Uimen-Lebed primitive island arc, oceanic islands and seamounts: the set of rocks is assumed to be formed in the Vendian. (2) A more evolved island arc comprising calc-alkaline volcanics and granites: a fore-arc trough in Middle-late Cambrian time was filled with disrupted products of pre-Middle Cambrian accretionary wedges and island arcs. (3) Collision of the more evolved island arc with the Siberian continent: folding, metamorphism and intrusion of granites occurred in late Cambrian-early Ordovician time. In the late Paleozoic, the above-mentioned Caledonian accretion-collision structure of the Siberian continent was broken by large-scale strike-slip faults into several segments. This resulted in the formation of a typical mosaic-block structure.

Geochemical and Nd‐isotopic systematics of the Permo‐Triassic Gympie Group, southeast Queensland

Nine subduction‐related magma suites are recognised from drillcore in the Upper Palaeozoic — Mesozoic volcanic‐intrusive sequence of the Gympie Group, part of the tectonically anomalous Gympie Province, southeast Queensland. Magmas of the Gympie Group include Lower Permian submarine (later subaerial) island‐arc tholeiites, basaltic tuff‐breccias and lavas comprising the Highbury Volcanics, as well as andesites and dacites in the unconformably overlying Rammutt Formation. Mantle‐derived mafic‐intermediate magmas also comprise several intrusive suites in the Gympie Group, some possessing alkaline and shoshonitic affinities. Most eruptives of the Highbury Volcanics and Rammutt Formation show uniformly high Nd‐isotope ratios [ϵNd(270 Ma) = +7.4 to +8.1] and geochemical features (e.g. high Zr/Nb, Sr/Nd and La/Nb) that reflect depleted asthenospheric mantle sources metasomatised by hydrous fluids from subducted lithosphere. Predominant andesites from the Rammutt Formation have higher Zr/Y, Nb/Y, Ti/V, La/Y, P/Nd and K/P than isotopically similar basalts of the Highbury Volcanics, as well as high Al2O3, Zr and SiO2. The andesites may represent island‐arc tholeiite magmas that assimilated as much as 40% isotopically primitive (young) terrigenous sediment, akin to turbiditic greywacke of Carboniferous accretionary complex origin in contiguous terranes of the New England Orogen. Rare dacites from the Rammutt Formation differentiated from island‐arc tholeiite magmas similar to those of the Highbury Volcanics without incorporation of substantial metasedimentary components. Lower initial Nd‐isotopic ratios characterise Gympie Group intrusive suites. In order of emplacement, these are: (i) the transitional‐alkaline multiphase Langton Dolerite sill (ϵNd = +6.2 to +6.7); (ii) dolerite dykes intimately associated with gold mineralisation (ϵNd = +4.3 to +5.7); and (iii) microdiorite dykes (ϵNd = +4.5 to +4.7) that include some near‐primary magmas with features transitional toward those of shoshonites. Low ϵNd(T) values (ϵNd = +4.9 to +5.4) are also shown by a unit of ankaramitic basalts from the top of the Highbury Volcanics. In addition to a predominantly depleted asthenospheric mantle source, isotopically enriched source component(s) are required for these low‐ϵNd magmas. Higher TiO2, Nb, Zr, Ti/V, Nb/Y and Nb/Zr for samples from the Langton Dolerite than for earlier formed lavas of the Gympie Group are also compatible with an enriched mantle contribution. High Zr/Y, La/Y, Ba/Zr, Th/Yb and La/Nb compared to (asthenosphere‐derived) backarc basalts from the contiguous Cambroon beds (ϵNd = +9 to +10), probably owe their origin to interaction of asthenospheric melts with older (isotopically evolved) subcontinental lithospheric mantle. A further component is required to account for distinctive Th–, K– and Si‐enrichments (e.g. Th/La up to 0.45) of the microdiorite‐shoshonite dykes and some primitive ankaramites and dolerite dykes. These enrichments are independent of uniform isotopic compositions and favour hybridisation of the mantle wedge source for these rocks by small‐volume, isotopically primitive K–Th–Si–rich partial melts, probably derived from eclogitic stalled slabs. The Gympie Province appears to have been the site of a continent — island‐arc accretion event in the Early to Middle Triassic. Geochemical and Nd‐isotopic compositions of volcanics from Gympie Group reflect the sequential involvement of asthenospheric and lithospheric sources attendant during the Late Palaeozoic — Early Mesozoic tectonomagmatic evolution of the palaeo‐Pacific Gondwana rim. Lower Permian eruptives of the Gympie Group were formed in a supra‐subduction zone (primitive island arc) setting, proximal to Gondwana rim accretionary elements, compatible with extension above a steepening Early Permian Benioff zone. Intrusive suites in the Gympie Group post‐date accretion of the Gympie Province to the eastern Australian continental margin by the Middle Triassic.

Archaean–Proterozoic transition: geochemistry, provenance and tectonic setting of metasedimentary rocks in central Fennoscandian Shield, Finland

The central part of the Fennoscandian Shield in Finland is composed of the Palaeoproterozoic Svecofennian domain and the Archaean Karelian craton with a Palaeoproterozoic allochthonous and autochthonous cover. A cryptic suture separating these areas and another tentative suture dividing the Svecofennian into central and southern parts have been proposed. The chemical composition of sedimentary rocks ( N=300) within the study area, including the effects of palaeoweathering, hydraulic sorting, depositional environment and post-depositional processes, have been studied in order to delineate sediment source components. The main proposed source components for the Archaean sedimentary rocks are weathered 3.0–3.2 Ga greenstone+granite±TTG and local 2.7 Ga sources. Autochthonous 2.2–1.9 Ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol (2.2–2.35 Ga), sedimentary rocks derived from the palaeosol, and mafic dykes and plateau volcanics (mainly 2.2–2.1 Ga) although in places locally derived non-weathered Archaean sources dominated. Archaean crust and 2.0–1.92 low-K bimodal rocks from a primitive island arc are the proposed source for the allochthonous Western Kaleva cover rocks. These formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone. The central Svecofennian sedimentary rocks can be divided into local arc-derived rocks (≤1.89 Ga) and older (≥1.91 Ga) rocks from a mixture of Western Kaleva sources and a 1.91–2.0 Ga mature island arc/active continental margin source. Rifting followed by increased subsidence during initial collision in the NE and subsequent arc reversal caused rapid erosion from the mountain belt, exposing diverse source compositions as seen in the large variation of Th/Sc (2–0.5), and deposition into an oblique hinterland basin further developing into a subduction related foredeep. Mature greywackes from the southern Svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline-affinity complexes and Archaean rocks. Less mature rocks also occur and had sources dominated either by island arc/active continental margin rocks or local picritic rocks. In the sedimentary record the Archaean–Proterozoic transition up to 2.1 Ga was dominated by input of mainly mafic plateau-type volcanic contribution to the Archaean detritus. Palaeoproterozoic sediments having a crustal component (≤2.1 Ga) show higher Th/Sc, Th/Cr, and lower Sm/Nd and Eu/Eu* relative to the Archaean rocks but locally low Th/Cr ratios complicate the situation. Ba depletion relative to K, Rb and Th is a characteristic feature of the sedimentary rocks of the central Fennoscandian Shield indicating high amounts of Ba lost from the clastic record during 2.3–1.9 Ga and further recycled back to the mantle forming a subduction component and an enriched mantle component. Ba depletion seems to have been especially characteristic of chemical weathering during 2.35–2.2 Ga under CO 2-rich and low-O 2 atmosphere. Whether this strong Ba depletion is characteristic of the Archaean–Proterozoic transition and quiet supercontinent stages in general remains to be determined.

Primitive CaO‐rich, silica‐undersaturated melts in island arcs: Evidence for the involvement of clinopyroxene‐rich lithologies in the petrogenesis of arc magmas

On the basis of the study of olivine‐hosted melt inclusions in a calc‐alkaline basalt from Batan Island (Philippines) we define a distinctive type of primitive, nepheline‐normative island arc magma characterized by unusually high CaO contents (up to 19.0 wt %) that cannot be simply explained by melting of the metasomatized peridotitic mantle wedge above subducting oceanic lithosphere. CaO‐rich melt inclusions with these characteristics are preserved in Fo85–90 olivine, and compositional variations among the inclusions are interpreted to reflect mixing between melts such as those found in the most CaO‐rich inclusions (present in Fo90 olivine) and melts similar to primitive “normal” island arc magmas (trapped in Fo85 olivine). Compilation of primitive island arc magmas from the literature shows that whole rocks and olivine‐hosted melt inclusions with CaO contents &gt;13 wt % are found in many arc volcanoes from all over the world in addition to Batan. These inclusions occur in lavas ranging from CaO‐rich ankaramites to basaltic andesites with low‐CaO contents (i.e., &lt;13 wt %). The globally occurring CaO‐rich inclusions and whole rocks comprise a group that although defined on the basis of their CaO contents is compositionally distinctive when compared to island arc lavas that have lower CaO contents; for example, they have lower FeO at a given SiO2 content than most arc lavas, and they are all nepheline normative, with normative nepheline contents positively correlated with CaO contents. Variations in CaO content and normative compositions of experimental partial melts of lherzolite related to changes of pressure, temperature, and source composition suggest that there are no conditions under which partial melting of peridotite can generate melts having CaO contents and other properties comparable to those observed for the primitive, CaO‐rich arc‐derived melts identified here. Although melting of peridotite at high pressure in the presence of CO2 can produce CaO‐rich, silica‐poor liquids, we consider it unlikely that this is responsible for producing the CaO‐rich, silica‐undersaturated melts considered in this study because there are significant differences in nearly all other compositional characteristics between the CaO‐rich arc magmas and melts known or thought to be produced by melting of carbonated peridotite. Model major element compositions of partial melts of clinopyroxene‐rich lithologies (mantle pyroxenites, lower crustal pyroxenites, and eclogites) calculated using the MELTS algorithm suggest that the most CaO‐rich, nepheline‐normative melt inclusions and whole rocks identified here could represent intermediate to high degree (∼10–40 wt %) partial melts of pyroxenites at lower crustal to upper mantle pressures. Such a hypothesis is supported by the comparison between the trace element compositions of model pyroxenite sources of the Batan CaO‐rich melt inclusions and naturally occurring pyroxenites. The most likely source of the primitive CaO‐rich, silica‐undersaturated arc melts identified here is lower crustal and shallow upper mantle pyroxene‐rich cumulates from arc environments because these cumulates have CaO concentrations at the upper end of the range observed for mantle pyroxenites. They are therefore more likely to yield partial melts with the restricted range of remarkably high CaO contents of the most CaO‐rich inclusions and whole rocks identified here. Moreover, these cumulates often contain amphibole, which would lower their solidus temperatures relative to the anhydrous pyroxenite equivalents to values more consistent with those expected in deep crustal or shallow subarc environments.

Age and geochemistry of basaltic complexes in western Costa Rica: Contributions to the geotectonic evolution of Central America

The age and origin of magmatic complexes along the Pacific Coast of Central America have important implications for the origin and tectonic evolution of this convergent plate margin. Here we present new 40Ar/39Ar laser age dates, major and trace element data, and initial Sr‐Nd‐Pb isotope ratios. The 124–109 Ma tholeiitic portions of the Santa Elena complex formed in a primitive island arc setting, believed to be part of the Chortis subduction zone. The geochemical similarities between the Santa Elena and Tortugal alkaline volcanic rocks suggest that Chortis block may extend south of the Hess Escarpment. The Nicoya, Herradura, Golfito, and Burica complexes and the tholeiitic Tortugal unit formed between 95 and 75 Ma and appear to be part of the Caribbean Large Igneous Province, thought to mark the initiation of the Galápagos hotspot. The Quepos and Osa complexes (65–59 Ma) represent accreted sections of an ocean island and an aseismic ridge, respectively, interpreted to reflect part of the Galápagos paleo‐hotspot track. An Oligocene unconformity throughout Central America may be related to the mid‐Eocene accretion of the Quepos and Osa complexes.

Growth and demise of an Archean carbonate platform, Steep Rock Lake, Ontario, Canada

The Steep Rock Group of northwest Ontario's Wabigoon subprovince is one of the world's thickest Archean carbonate platform successions. It was deposited unconformably over a 3001-2928 Ma gneissic terrane, and contains a remarkable group of biogenic and oolitic limestones, dolostones, micrites, and karst breccias capped by a thick paleosol developed between and over karst towers. The presence of aragonite fans, herringbone calcite, and rare gypsum molds suggests that the carbonate platform experienced at least local anaerobic and hypersaline depositional conditions. This sequence shows that a combination of chemical and biological processes was able to build a carbonate platform 500 m thick by 3 billion years ago. The carbonate platform is structurally overlain by a mixture of complexly deformed rocks of the Dismal Ashrock forming a mélange with blocks of ultramafic volcaniclastic rocks, mafic volcanics, carbonate, tonalite, lenses of Fe-ore rock, and metasedimentary rocks, in a shaly, serpentinitic, and fragmental ultramafic volcaniclastic matrix. The mélange shows evidence of polyphase deformation, with early high-strain fabrics formed at amphibolite facies, and later superimposed brittle fabrics related to the final emplacement of the mélange over the carbonate platform. An amphibolite- through greenschist-grade shear zone marks the upper contact of the mélange with overlying mafic volcanic and tuffaceous rocks of the ca. 2932 Ma Witch Bay allochthon, interpreted as a primitive island arc sequence. We suggest an evolutionary model for the area that begins with rifting of an arc sequence (Marmion Complex of the Wabigoon arc) that initiated subsidence and sedimentation on the Steep Rock platform and its correlatives that extend for a restored strike length exceeding 1000 km. Shallow water carbonate sedimentation continued until the platform was uplifted on the flanks of a flexural bulge related to the approach of the Witch Bay allochthon, representing collision of the rifted arc margin of the Wabigoon subprovince with the Witch Bay arc. Mélange of the Dismal Ashrock was formed as off-axis volcanic rocks were accreted to the base of the Witch Bay allochthon prior to its collision with the Steep Rock platform.

Geology of the Yozgat Region and Evolution of the Collisional Cankiri Basin

The Cankiri Basin evolved as a detrital basin during the Late Tertiary in Central Anatolia between the Pontides (Sakarya continent) and the Kirsehir block. It is E-W trending, and along its southern border it overlies the Kirsehir block unconformably. The Kirsehir block consists of two tectonic belts. Structurally, the lower unit is the Kirsehir Massif, consisting of a platform sequence and a marble association; the upper unit is the Cicekdag belt. The Cicekdag belt consists of thick mafic volcaniclastic rocks and interbedded sediments and rare gabbro masses. This belt probably was developed as a primitive island-arc in the Neotethys ocean. During the Late Cretaceous period, the Cicekdag belt was thrust southward onto the Kirsehir platform. Consequently, the overburden of the nappes caused metamorphism of the Kirsehir platform and the lower parts of the Cicekdag belt. The Yozgat magmatics, consisting of granitic plutons and dacitic rhyolitic lavas, intruded into the Kirsehir block, which was formed by the...

The Rattlesnake Creek terrane, Klamath Mountains, California: An early Mesozoic volcanic arc and its basement of tectonically disrupted oceanic crust

The Rattlesnake Creek terrane in the south-western Klamath Mountains consists of a serpentinite matrix melange basement unconformably overlain by a coherent cover sequence of Upper Triassic and Lower Jurassic volcanic, hemipelagic, and clastic sedimentary rocks. Both the meelange and cover sequence are intruded by a suite of ca. 193-207 Ma (U-Pb zircon) gabbroic to quartz dioritic plutons. These rocks collectively record a complex, multistage evolution through several distinct tectonic environments. Blocks in the meelange basement include peridotite, greenstone, amphibolite, pillow basalt, various generally mafic plutonic rocks, metachert, and limestone, all suspended in a matrix of sheared serpentinite. Trace element chemistry indicates that basalt, amphibolite, and greenstone blocks were derived from normal to enriched mid-oceanic-ridge basalt (N-MORB to E-MORB) and within-plate basalt (WPB) protoliths. There is no geochemical evidence that any blocks were derived from a magmatic arc, nor do any of the sedimentary rocks contained within the meelange reflect deposition near a terrigenous sediment source. Collectively these relations indicate that the basement meelange was derived by disruption of oceanic crust and upper mantle in a setting far removed from either an arc or a terrigenous landmass and are most consistent with an interpretation that meelange formation oc-curred in an oceanic fracture zone. The overlying cover sequence is divisible into a stratigraphically lower Salt Creek assemblage and a stratigraphically higher Dubakella Mountain assemblage, both deposited in a marine setting. Volcanic rocks in the lower assemblage are basaltic pillow lava and pillow breccia with the trace element characteristics of a primitive island arc tholeiite suite, whereas those in the upper assemblage are predominantly chnopyroxene-phyric basalt to basaltic andesite and compositionally similar volcaniclastic rocks, with the geochemical characteristics of a more evolved (cale-alkaline to shoshonitic) volcanic arc suite. The geochemical character of these volcanic rocks is indistinguishable from that of the coeval early Mesozoic plutons, leading to the conclusion that these plutons represent the intrusive roots of the volcanic succession. Collectively, these rocks reflect construction of a Late Triassic to Early Jurassic island arc and indicate that subduction had initiated within or near the under-lying fracture zone assemblage by at least the late Triassic. The latest stages of crustal disruption in the basement thus probably occurred in a subduction zone environment. Both cover sequence units also contain inter-layered chert and argillite, as well as coarse epiclastic rocks with an abundance of quartzose, sedimentary, and metasedimentary grains. These elastic rocks provide the first evidence for terrigenous input into the Rattle-snake Creek terrane and indicate that the early Mesozoic arc developed in proximity to a terrigenous sediment source, probably Paleozoic rocks of the western North American Cordillera. Several lines of evidence suggest that early Mesozoic arc magmatism was accompanied by an episode of extensional tectonism. These include structural evidence for an early Mesozoic phase of high-angle faulting that appears to have localized cover sequence deposition in fault-bounded grabens, sedimentologic evidence indicating that both the basement and terrigenous sediment sources experienced episodic pulses of uplift and erosion during cover sequence deposition, and widespread intrusion of arc plutons into the upper crustal levels of the are carapace. Both the basement complex and the early Mesozoic arc assemblage of the Rattlesnake Creek terrane were then deformed by thrust faulting, folding, and penetrative foliation development during over-thrusting from the east by the western Hayfork terrane in the Middle Jurassic. Regional de-formation was in response to northeast-south-west-directed shortening and was accompanied by metamorphism to subgreenschist grade. Insofar as the western Hayfork terrane represents part of a Middle Jurassic arc whose plutonic roots intrude most Klamath Mountain terranes east of and including the Rattle-snake Creek terrane, it is clear that the Rattlesnake Creek terrane formed part of the Klamath Mountain province by Middle Jurassic time. The paleogeographic and tectonic relation of the Rattlesnake Creek terrane to the rest of the Klamath Mountains prior to the Middle Jurassic, however, is unclear.