Intra-oceanic Island Arc Research Articles

Tectonic setting and paleoenvironment of terranes in the southern New England orogen, eastern Australia as constrained by radiolarian biostratigraphy

Radiolarians are abundant in the Gamilaroi, Djungati and Anaiwan terranes of the New England orogen in eastern Australia. These microfossils present the first age constraints on the timing of development of hitherto undated lithologies within the various juxtaposed terranes of the orogen. They give a biostratigraphic framework for interpreting the history of marine sedimentation in these terranes and have significant implications for published tectonic models. Radiolarians also show that the stratigraphically lowermost rocks of the Gamilaroi terrane are most probably of Devonian age. Better stratigraphic resolution permits interpretation of these rocks as part of an intra-oceanic island arc succession which accreted to the eastern margin of Australia (Gondwana) around the end of the Devonian. This intra-oceanic arc is distinct from a younger, Carboniferous, superposed continental arc sequence which constitutes a successor basin that developed over the Gamilaroi terrane subsequent to its accretion to the margin of Australia. Radiolarians provide the basis for dating siliceous lithologies which dominate the Djungati terrane and permit a more detailed analysis of the history of this terrane. During the middle Silurian through Late Devonian the Djungati terrane was part of an oceanic basin which was isolated from any source of terrigenous sedimentation. The Djungati terrane was subsequently influenced by volcanic island arc activity and was tectonically disrupted during the latest Devonian to Early Carboniferous. Radiolarian age data show that age correlations inferred, on the basis of similarities in detrital sandstone petrography, between lithostratigraphic units in the Gamilaroi and Djungati terranes, are not always appropriate. Djungati terrane has been widely interpreted as a subduction complex related to the Gamilaroi terrane. Radiolarian data now elucidate much of the structural complexity of this terrane. Significant differences between the Djungati terrane and the style of well-documented subduction complexes include the close spacing of zones of radiolarian chert, the dominance of this lithology over others and the lateral extent of many of the chert horizons. The Anaiwan terrane has features which are characteristic of many well-documented subduction complexes. Radiolarians are abundant in this terrane and can be used to show that it developed along the eastern margin of Australia in response to Early Carboniferous subduction of Late Devonian to Early Carboniferous oceanic crust. Packages of chert accreted into the subduction complex are progressively younger towards the northeast indicating that subduction was directed to the southwest.

ソ連カムチャツカ半島東北部オリュートル地域の深成岩類

Geology and petrology of three plutons (the Epilchik, Machevna and Tigil) in the southern Olyutor Range of Northeastern Kamchatka (USSR) are briefly summarized in this paper. The Upper Cretaceous volcano-sedimentary rocks in the range consititute a prolongation of the submarine Shirshov ridge, that separates the Aleutian passive back arc basin from the Komandorsky basin. Many ultramafic and mafic to intermediate plutons of Paleogene age are distributed in the Upper Cretaceous rocks. The Epilchik ultramafic pluton shows a concentric structure: the dunitic core passes progressively into wehrlite and clinopyroxenite of the rim, and into a discontinous margin of gabbro. The common presence of diopside and chromian spinel and the absence of orthopyroxene in the ultramafic pluton are characteristics recognized universally in Alaskan-type ultramafic complexes. The Machevna pluton comprises North and South plutons. The two plutons are both zoned, with gabbroic rim and quartz dioritic core, accompanied by a small amount of clinopyroxenite. The northeastern contact of the North Machevna pluton is characterized by a complicated zone of gabbro and basalt, both the rocks have metamophosed to the amphibolite facies, presumably by synshear, hydrothermal fluid infiltration. The Tigil pluton is composed mainly of gabbro and quartz gabbro. The constituent rocks of the Machevna and Tigil plutons are characterized by low abundances in Ti, Zr and Rb, and by high magnetic susceptibility (30-200×10-3SI unit). These features are similar to those of plutonic rocks formed in intraoceanic island arcs, such as the Aleutian arc and Papua New Guinea etc.. The constituent rocks of the three plutons and their country rocks have suffered pervasive low-grade metamorphism, commonly up to the greenschist facies. To clarify the petrogeneses of three plutons in detail, further studies from both sides of igneous and metamorphic petrology are wanted.

Petrotectonic characterization of the Central Andean Terrane, Colombia

The Central Andean Terrane is a lower Paleozoic metavolcanoclastic-metapelitic belt that extends from the Central Andes of Colombia to northern Peru. On a section across the Central Cordillera of Colombia, the terrane consists mainly of amphibolitic schists and graphitic schists that range from greenschist facies to epidote-amphibolite facies. The mineral assemblages depict one prograde metamorphic event. Schists are isoclinally folded and, toward the western margin of the terrane, foliation is transposed. This transposition is probably related to overthrusting of the Amaime Terrane, which lies west of the Central Andean Terrane. Major and trace element geochemistry reveals two distinct sources for the terrane's protolith: an intra-oceanic island arc, and a continental margin. Accretion and metamorphism of the terrane took place in Late Silurian-Early Devonian time. Based on current Paleozoic reconstructions, two models of tectonic evolution of the Central Andean Terrane are presented here, both involving closure of a back-arc basin. The first model is within an Andean-type margin setting and the second is within a continental collision setting.

A crustal section of an intra-oceanic island arc: The Late Jurassic-Early Cretaceous Guanajuato magmatic sequence, central Mexico

The Late Jurassic-Early Cretaceous Alisitos-Teloloapan intra-oceanic arc, which extends along the Pacific coast of Mexico, is overlain by the extensive thick calc-alkaline Tertiary volcanics of the Sierra Madre Occidental. In central-southern Mexico this arc can be divided in to two major units. Its base, which is mainly exposed in the Guanajuato-Leon area, consists of a volcano-plutonic assemblage refered to as the Guanajuato magmatic sequence. The top consists of pillowed calc-alkaline basalts and andesites overlain and/or interbedded with a thick volcaniclastic sedimentary pile in which lenses of reworked Albian-Aptian reefal limestones are interlayered. The Guanajuato magmatic sequence represents the crustal section of an intra-oceanic nascent island arc and from top to base is made up of: (1) volcaniclastic sedimentary cover, (2) a predominantly pillowed basaltic sequence, (3) a doleritic and basaltic dyke complex, (4) silicic plutonic rocks and salite diorite, (5) Mg-hornblende diorite, (6) layered cumulate gabbros and, (7) ultramafic-mafic cumulates. Plutonic rocks of intermediate to silicic composition are volumetrically abundant. All the rocks have suffered low-grade metamorphism with magmatic textures preserved. Metamorphic textures do not occur. Mg-rich olivine is restricted to the ultramafic cumulates, whereas Ca-rich clinopyroxene is present in all the rock types, although with the exception of the most differentiated silica rich plutonic rocks. The textural relationships and mineral chemistry of the clinopyroxenes and hornblende in the ultramafic cumulates and in the clinopyroxene gabbros point to an origin by igneous accumulation from a tholeiitic parent magma at shallow levels above the mantle-crust boundary. Al-rich basalts and dolerites belong to a low-K tholeiitic suite. The Nb contents, which are abnormaly high for arc tholeiites, are in the range of N-MORB. Such enrichments have been related to an immature stage of the arc development, the mantle wedge being still enriched. This is in agreement with the evidence that the Guanajuato tholeiitic magma represents the earliest products of the Alisitos-Teloloapan arc magmatic activity. The Guanajuato basic rocks and hornblende diorite show similar high ε Nd ratios which fall in the range of the values of oceanic arcs. The most siliceous plutonic rocks differ from the basic suite by virtue of their slightly lower ε Nd and higher ε Sr ratios, which may be explained by an assimilation-fractional crystallization (AFC) process. The siliceous melts differentiated by crystal fractionation from the tholeiitic mantle magma and assimilated material such as metamorphosed oceanic crust and sediments, which form the wall rocks of the magma chamber. The high ε Sr ratios of the basalts and their feeder dykes are probably related to the polyphase low-grade metamorphism (i.e. hydrothermal oceanic alteration and regional metamorphism) that has affected the uppermost levels of the arc.

The geology and tectonic evolution of Waigeo Island, NE Indonesia

Waigeo occupies a critical position between the Halmahera-Philippine arcs to the northwest and Australia-New Guinea to the southeast. The island consists of a deformed ophiolitic basement of supra-subduction zone type overlain by probable Paleogene forearc sedimentary rocks. The forerarc basement is cut by mylonite shear zones and the basement and sedimentary cover rocks were deformed by southward-directed thrusts and associated folds during the Oligocene. The deformed sequences are intruded by basic dykes, thought to be coeval with basalts and andesites of Late Oligocene age. The entire Paleogene sequence is overlain unconformably by a thick (up to 2000 m) sequence of Miocene limestones which accumulated during a tectonically quiet period. A final phase of deformation occurred during the Pliocene which caused the development of two very large anticlines and an intervening syncline, associated with left-lateral wrench faults. Waigeo is interpreted to have been situated in a forearc position in an intra-oceanic island arc during the early Paleogene, forming part of the East Halmahera-Waigeo forearc terrane. The Waigeo arc terrane collided with a continental block in about the Middle Oligocene, contemporaneous with similar arc-continent collision in northern New Guinea. A period of tectonic quiescence during the Miocene was followed by Pliocene deformation in Waigeo related to a left-lateral wrench faulting on splays of the Sorong Fault in northern New Guinea. The Pliocene deformation is interpreted as resulting from compression on a right-stepping restraining bend in this wrench fault system.

Structural development of Sumisu Rift, Izu‐Bonin Arc

Geophysical swath mapping, multichannel seismic profiling, and ocean drilling data are used to document the structural evolution of Sumisu Rift and to analyze the pattern of strain resulting from extension of an intraoceanic island arc. The ∼120‐km‐long, 30–50‐km‐wide Sumisu Rift is bounded to the north and south by structural and volcanic highs west of the Sumisu and Torishima calderas and longitudinally by curvilinear border fault zones with both convex and concave dip slopes. The zig‐zag pattern of normal faults (average strikes 337° and 355°) indicates extension oriented 076°±10°, orthogonal to the volcanic arc. Three oblique transfer zones divide the rift along strike into four segments with different fault trends and uplift/subsidence patterns. Differential strain across the transfer zones is accommodated by interdigitating, rift‐parallel faults and sometimes by cross‐rift volcanism, rather than by strike‐ or oblique‐slip faults. From estimates of extension (2–5 km), the age of the rift (∼2 Ma), and the accelerating subsidence, we infer that Sumisu Rift is in the early synrift stage of back arc basin formation. Following an early sag phase, half graben formed with synthetically faulted, structural rollovers facing large‐offset (2–2.5 km throw) border fault zones. In the three northern rift segments the largest faults are on the arc side and dip 60°–75°W, whereas in the southern segment they are on the west side and dip 25°–50°E. The present “full graben” stage is dominated by hanging wall antithetic faulting, basin widening by footwall collapse, and a concentration of subsidence in an inner rift. The hanging wall collapses, but not necessarily as a result of border fault propagation from adjacent rift segments. Whereas the border faults may penetrate the Theologically weak lithosphere (Te ≈ 3 km), many of the hanging wall and footwall collapse structures are detached only a few kilometers below the seafloor. Back arc volcanism, usually erupted along faults, occurs in the rift and along the protoremnant arc during both stages. Where drilled, the arc margin has been uplifted 1.1±0.5 km concurrently with ∼1.1 km of rift basin subsidence. Extremely high sedimentation rates, up to 6 m/kyr in the inner rift, have kept pace with synrift faulting, created a smooth basin floor, and resulted in sediment thicknesses that mimic the differential basin subsidence. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. No obvious correlations are observed between the rift structures and preexisting cross‐arc trends.

Petrological and geochemical constraints on the genesis of Mesozoic–Cenozoic magmatism of King George Island, South Shetland Islands, Antarctica

Petrological and geochemical data are reported for a series of Late Cretaceous-Middle Miocene volcanic, hypabyssal and intrusive rocks from King George Island (KGI) and from nearby Ridley Island, South Shetland Islands. Major element data indicate a calc-alkaline, basic to intermediate composition for the analysed samples. Although emplaced on a continental margin, the KGI rocks generally display low abundances of incompatible trace elements, close to those typically observed in calc-alkaline suites erupted in intraoceanic island arcs. A few samples have a significant negative Ce anomaly. Many incompatible elements define smooth positive trends on interelemental variation diagrams which suggests that magmas erupted at different times on KGI maintained a rather constant composition in terms of incompatible element ratios. Geochemical modelling, based on Sr isotope ratios and incompatible element ratios, suggests that the primary calc-alkaline magmas of KGI were all generated in an upper mantle modified by addition of small amounts of pelagic sediments dragged down by subduction processes.

On the origin of the compositional variations of the post-collisional granitoids in arc-terranes and suture zones, Eastern Desert, Egypt

The Pan-African (Late Precambrian) basement complex of the Eastern Desert of Egypt is thought to be the product of immature intra-oceanic island arcs, mature intra-oceanic island arcs, and syn- and post-collision crustal anatexis. Aswan granitoids intruding the southern suture zone, Ras Gharib granitoids and the subvolcanic peralkaline granites of Gabal Nuqara intruding the northern arc-terrane of the Eastern Desert of Egypt are selected for a comparative study of post-collision granitoids. AFM variation diagram and Peacock's 1931 alkali-lime index revealed the calc-alkaline character of the “granodiorite-adamellite-leucogranite suite”. Petrochemical variation diagrams show that the Aswan granitoids are relatively enriched in K 2O and FeO ∗; whereas the Ras Gharib granitoids are relatively enriched in CaO. They plot dominantly in the field of granites on Ab-An-Or diagram of Barker 1979, suggesting crustal anatexis for their magma source, in contrast to subduction-related magmas which are usually dominated by diorites or tonalites. The minor peralkaline granites, which are synchronous with the calc-alkaline post-collision granites, are not enriched in Nb, Zr and REE as typical anorogenic granites, which suggest that they may be formed as a late phase of the post-collision granites by gas-transfer processes. Assimilation of schists and gneisses rich in K and Fe in micas, which are common in the suture zones; and dioritetonalite rocks, rich in CaNa plagioclases, which characterize the arc-terranes may modify the primarily “near-eutectic” granitic magma locally in some granite intrusions forming “contaminated” or “hybrid” granitoids of the granodiorite-adamellite-leucogranite suite.

伊豆弧と古伊豆弧の発達とそのテクトニクス

The Izu-Ogasawara (Bonin) arc is along the eastern boundary of the Philippine Sea plate, where the Pacific plate is subducted from east. It is one of the typical intra-oceanic island arcs, and consists of the following units from the oceanic side; the “forearc ophiolite”, forearc made of volcanic island arc of old age, active island arc volcanics, rift (“back-arc depression”), and back arc basin with several inactive volcanics. On land in the southern Kanto to the Southern Fossa Magna regions and surroundings, there are many lines of evidence for the ancient relicts of the Paleo-Izu materials and other oceanic materials, which have been accreted by various ways since Oligocene.Before the accretion of island arc materials, ophiolitic rocks, composed chiefly of MORB with alkali basalt and harzburgite, were jammed within the terrigenous accretionary complex in the Shimanto belt. These rocks might be the obducted materials from the Pacific side. After that abundant volcanic island arc suites have been accreted to Honshu, sometimes mixed with Honshu-derived terriginous materials, sometimes island arc materials only. The Fossa Magna might be formed between the stages of ophiolite obduction and island arc com-mencement, around 17 MaBP. Several accretionary prisms composed of the Izu forearc mate-rials are developed in the present Miura-Boso Peninsulas. The volcanic arc materials, consisting remarkably of ash or lapilli fall deposits near the volcanic front, were offscraped and accreted to the Honshu arc side on the Sagami Trough. This reflecs a particular tectonics around the trench-trench-trench triple junction off the Boso Peninsula, which arrived at around the present position at about 17 MaBP.

Comments on a Paper by Jon. D. Woodhead Titled 'The Origin of Geochemical Variations in Mariana Lavas: a General Model for Petrogenesis in Intra-oceanic Island Arcs?'

Comments on a Paper by Jon. D. Woodhead Titled 'The Origin of Geochemical Variations in Mariana Lavas: a General Model for Petrogenesis in Intra-oceanic Island Arcs?'

A geochemical approach to allochthonous terranes: a Pan-African case study

The recognition of Mesozoic and Cenozoic terranes can best be made from palaeomagnetic, structural and palaeontological studies, but older regions of continental crust require geochemical constraints to evaluate crustal growth through terrane accretion. For Precambrian shields, the pattern of Pb and Nd isotopic provinces may reveal the mechanism of crustal growth. The Afro-Arabian Shield was generated by calc-alkaline magmatism between 900 and 600 Ma ago. This example of Pan-African crustal growth underlies an area of at least 1.2 x 10 6 km 2 , which may extend to 3.5 x 10 6 km 2 beneath Phanerozoic sediments and Tertiary volcanic cover. Field evidence and trace element geochemistry suggest that Pan-African tectonics began as a series of intra-oceanic island arcs that were accreted to form continental lithosphere over a period of 300 Ma. The great majority of Nd and Pb isotope ratios obtained for igneous rocks from the shield are indicative of a mantle magma source. Although many of the dismembered ophiolites cannot be identified with inter-terrane sutures in their present location, the eastern margin of the Nabitah orogenic belt is a major tectonic break that coincides with a critical boundary between Nd and Pb isotopic provinces and is marked by a linear array of ophiolite fragments across the length of the shield. Other terrane boundaries have not been identified conclusively, both because coeval island arcs can not be distinguished readily on isotopic grounds and because many ophiolites are allochthonous. However, the calculated rates of crustal growth (measured as volume of magma, extracted from the mantle per unit time) between 900 and 600 Ma are similar to those calculated for Phanerozoic terranes from the Canadian Cordillera. Such high rates in the Afro-Arabian Shield suggest that island arc terranes have accreted along a continental margin now exposed in NE Africa, together with minor continental fragments. If crustal growth rates during this time were no greater than contemporary rates, ca . 4000 km of arc length are required, which is considerably less than that responsible for crustal growth in the SW Pacific.

Mass balance calculations for two sections of island arc crust and implications for the formation of continents

The bulk compositions of two exposed sections of intra-oceanic island arc crust (the Talkeetna volcanics and Border Ranges ultramafic-mafic complex in southeastern Alaska, and the Canyon Mountain Complex in northeastern Oregon) represent basalt ( MgO = 11%) and probably basaltic andesite ( MgO = 8%), respectively. Both have REE abundances approximately 10 times chondrite, with no LREE-enrichment. These compositions, determined by mass balance based on the relative thicknesses of lithologic units in the exposed sections, are reasonable parental magma compositions based on Fe/O *MgO and Ni contents of olivine in the basal crustal cumulate rocks and the lack of significant Eu anomalies. Accretion of arcs such as these is inappropriate as a sole mechanism for generating post-Archean continental crust, since the latter is significantly LREE-enriched and is generally considered to be andesitic. Thus, if these arcs represent a major ingredient in Proterozoic and younger continental crust, their bulk compositions need to be modified. A combination of processes may be invoked, including (a) delamination of basal cumulates to make the sections less mafic, (b) addition of an alkalic component to enrich the crust in LREE and other lithophile elements, and (c) partial melting of the lower crust combined with delamination of the residue to do both. The basal parts of these crustal sections represent potential mantle heterogeneities.

Comparison between two Palaeozoic island-arc terranes in northern California (eastern Klamath and northern Sierra Nevada): geodynamic constraints

The Palaeozoic geodynamic environment of northern California is considered to be one of offshore island arcs bounded by marginal seas. The petrological and geochemical features of the eastern Klamath and northern Sierra Nevada island-arc sequences are important indicators of the geodynamic evolution of the Cordillera. The eastern Klamath pre-Early Devonian island-arc sequence consists of pillowed basalts andesites and boninites, overlain by low-K rhyolites, both suites erupted in a submarine environment. These volcanics, characterized by almost flat REE patterns and high ϵ Nd represent a primitive intraoceanic island arc. The mid-Palaeozoic volcanic succession of northern Sierra Nevada is formed of calc-alkaline rhyolites conformably overlain by either calc-alkaline (in the north) or low-K tholeiite (in the south) basalts and andesites. The volcanic episode ended in the Early Carboniferous with a differentiated shoshonitic suite. All the massive or pillowed flows are interbedded with volcaniclastic sediments, suggesting that they were erupted partly in a shallow-water environment. The geochemical features of the volcanics, moderate to high LREE enrichment and low ϵ Nd values, are evidence for the involvement of a crustal component in the magma genesis. The mid-Palaeozoic volcanic sequence therefore represents a continent-based island arc. This microcontinent could be the Sonomia microplate (excluding the eastern Klamath plate). Thus, these two northern Californian Palaeozoic sequences cannot be considered as lateral counterparts but could represent the remnants of oceanic and continental island arcs as observed in the present-day East Pacific areas along the Eurasiatic margin.

Geochemistry of the Mariana arc (western Pacific): Source composition and processes

The northern Mariana islands in the western Pacific form a prime example of an intra-oceanic island arc system, free from the effects of continental crustal contamination. Radiogenic (Sr, Nd, Pb, Hf) and stable (O, H, S) isotope data are presented for a suite of lavas from these islands, together with rare-earth and other trace-element compositional data for the same samples. This geochemical synthesis provides an opportunity to address a number of outstanding questions concerning oceanic arc petrogenesis, in particular the nature of the mantle source and processes occurring within this region. Much debate surrounds the origin of the commonly observed enrichments in island arc lavas relative to mid-ocean ridge basalt (MORB), in particular whether these result from incorporation of slab-derived components into a MORB-type mantle or whether they reflect preferential melting of a source with affinities to oceanic island basalt (OIB). Although some geochemical data are ambiguous, the majority appear to favour the slab involvement hypothesis, with contributions from both subducted sediment and altered oceanic crust. The trace-element enrichment patterns of the Mariana arc volcanics suggest that the agent of mass transfer between the subducting slab and overlying mantle wedge is a fluid, which has leached elements from the slab, rather than a direct partial melt of the latter. Although the exact composition of this fluid is unknown, the data indicate that a large proportion of it may be aqueous. Elemental partitioning into this fluid is highly selective (i.e. mass transfer between the slab and mantle wedge is not an iso-chemical process), a conclusion which has considerable implications for studies of crust and mantle evolution. The commonly cited depletions of high-field-strength elements (HFSE) in island arcs relative to MORB are thought to be largely a product of the data normalization process; in reality substantial overlap exists between HFSE concentrations in arc lavas and MORB. Regional geochemical variations between the islands in the arc are small and are thought most likely to be related to heterogeneity in the nature of the subducting assemblage.

Radioactivity and geochemistry of calc-alkaline neovolcanites of the West Carpathians

The distribution of radioactive elements(Th, U, K) and selected trace elements(Rb, Sr, Ba, Y, Zr, V, Cr, Ni) in a set of 29 volcanic rocks of the West Carpathians was studied. The rock types follow the typical calc-alkaline sequence from basaltic andesites to rhyolites with average contents of SiO2, K2O, Th, U, Rb, Ba and Zr increasing, and with average contents of TiO2, Al2O3, FeOt, MnO, MgO, CaO, P2O5, Sr, V, Cr and Ni decreasing from less to more acid rocks. Statistically significant differences in the contents of Th, U, Rb, Sr, Ba, Zr, K2O, CaO and P2O5, found between the pyroxene andesites of Central and Eastern Slovakia, may indicate that the volcanism in both regions took place under different conditions. The andesites of Central Slovakia are characterized by relatively high contents of Th, U and Rb as opposed to the andesites from other volcanic areas, e.g., from the intraoceanic island arcs, the Andes or the Mexican volcanic belt.

Geochemistry and tectonic interpretation of some metavolcanic rock units of the western North Cascades, Washington

Metavolcanic rocks of the western North Cascades of Washington, including the Chilliwack Group (CG), the Elbow Lake Formation (ELFm), and the Yellow Aster Complex (YAC), have been analyzed for clinopyroxene composition and whole-rock major, trace, and rare-earth elements (REE9s) to determine their original chemistry and tectonic setting. Covariant trends, elemental abundances, and ratios of high field-strength elements (HFSE9s) and REE9s indicate that the previously correlated CG and ELFm, and possibly equivalent basalts of the YAC, are separate rock units which evolved in distinct tectonic settings. Volcanic flows within the upper part of the upper Paleozoic CG are composed of tholeiitic basalt with lesser basaltic andesite and minor dacite. The CG originated as an intraoceanic island arc. Volcanic rocks of the ELFm are largely alkalic basalt, distinguished from the CG by HFSE enrichment and low Y/Nb and Zr/Nb. The ELFm is interpreted to represent structurally dismembered oceanic islands of Pennsylvanian to Jurassic age. An affinity with the Cache Creek terrane is suggested by correlation with the Orcas Chert of the San Juan Islands, which bears a Tethyan fauna. Basalt dikes of the YAC are of continental affinity, perhaps produced during an episode of continental rifting. High Cr and Ni contents, large-ion lithophile element enrichment, and transitional REE patterns distinguish YAC dikes from the CG and ELFm.

Upper Jurassic mafic magmatic rocks of the eastern Klamath Mountains, northern California: Remnant of a volcanic arc built on young continental crust

Diabasic and gabbroic dikes intruding the lower Paleozoic Trinity Ophiolite in the Lovers Leap section, Klamath Mountains, California, display strong calc-alkalic petrological and geochemical features (occurrence of primary amphiboles, zoned plagioclase phenocrysts and biotite, low TiO 2 , high incompatible trace-element contents, and light rare earth element enrichment). These dikes, of Late Jurassic age (149 ±6 Ma by K-Ar), are petrographically and geochemically similar to the contemporaneous calc-alkalic ultramafic-mafic magmatism well developed through the Klamath Mountains. They present negative Nb, Zr, and Ti anomalies typical of subduction-related magmatism and probably belong to a volcanic arc on an active continental margin. Their ϵ Sr (between -9.7 and -12.5) and ϵ Nd (between 5.6 and 6.3) values compare with some western U.S. Mesozoic granites. The Nd isotopic values, lower than those of mid-oceanic ridge basalts and intra-oceanic island arcs, suggest that these dikes, deriving from a depleted mantle source, have been slightly contaminated by continental material, probably subducted sediments. Values of ϵ Nd suggest, moreover, that no old continental crust underlies the Klamath Mountains.

The Origin of Geochemical Variations in Mariana Lavas: A General Model for Petrogenesis in Intra-Oceanic Island Arcs?

The Origin of Geochemical Variations in Mariana Lavas: A General Model for Petrogenesis in Intra-Oceanic Island Arcs?

Opening of the Grenada back-arc Basin and evolution of the Caribbean plate during the Mesozoic and early Paleogene

Geological and geophysical data indicate that the Grenada Basin was presumably created, during the Paleocene, by sundering of a proto-Eastern Caribbean arc into a remnant arc to the west (Aves Swell) and an active arc to the east (Lesser Antilles Ridge). Grenada Basin spreading is thought to have been penecontemporaneous with the creation of the Yucatan Basin located at the opposite side of the Caribbean Sea. I suggest that a continuous Mesozoic Caribbean Arc (M.C.A.), including the Greater Antilles, the Aves-Lesser Antilles system, and the Aruba-Blanquilla Chain (Netherland-Venezuelan Antilles), was initiated in the Pacific, probably about 130-120 Ma ago. Its arrival in front of, and its subsequent motion inside the Central Atlantic (“Tethyan”) seaway caused the opening of both Yucatan and Grenada basins which occurred at the two initial points of contact with the North and South American cratons. In contrast to the style of many other island arcs, this back-arc spreading event occurred only once in the long history of the M.C.A. The Lesser Antilles appear to be the oldest currently active intra-oceanic island arc.

Precambrian ophiolites of arabia: geologic settings, UPb geochronology, Pb-isotope characteristics, and implications for continental accretion

Disrupted ophiolites occur in linear belts up to 900 km long between microplates that collided during the late Proterozoic to form the Arabian Shield. UPb zircon ages and Pb-isotope data from these ophiolitic rocks help constrain the history of accretion of the Arabian Shield and thereby contribute to the definition of its microplates and terranes. Terranes of the central and western Arabian Shield are generally thought to represent intraoceanic island arcs that range in age from about 900 to 640 Ma; however, a region of the eastern Arabian Shield contains rocks of Early Proterozoic age and may represent an exotic continental fragment entrained between the arc complexes. Ophiolites of the Yanbu suture (northwestern shield), dated by UPb (zircon) and SmNd (mineral isochron) methods, yield model ages of 740–780 Ma. These are among the oldest well-dated rocks in the northwestern Arabian Shield. Ages from the Jabal al Wask complex overlap with ages of adjacent arc rocks. This overlap in age supports geologic and geochemical evidence that the Wask complex represents a fragment of back-arc oceanic lithosphere formed during arc magmatism. Older ages of about 780 Ma for gabbro from the Jabal Ess ophiolite suggest that the ophiolite is either a fragment of fore-arc oceanic crust or oceanic basement on which an arc was built. Gabbro samples from ophiolites of the Bir Umq suture (west-central Arabian Shield) yield zircons with ages of 820–870 Ma and $1250 Ma. The 820–870 Ma dates overlap with ages of the oldest nearby arc rocks; this favors an intra-arc or near-arc paleotectonic setting. The older zircons suggest that middle or early Proterozoic crustal material, possibly derived from the Mozambique belt of Africa, was present during back- or intra-arc magmatism. Plagiogranite from the Bir Tuluhah ophiolitic complex at the nothern end of the 900 km-long Nabitah mobile belt was dated by the zircon UPb method at ∼ 830 Ma. This date is in the range of the oldest dated arc rocks along the northern and central parts of the Nabitah suture, but is ∼ 100 Ma older than the oldest arc plutons (tonalites) associated with the southern part of the belt. These age relations suggest that the northern part of the Nabitah belt contains an extension of the Bir Umq suture that was transposed parallel to the Nabitah trend during collision of the arc terranes of the northwest Arabian Shield with the Afif plate to the east. Feldspar lead-isotope data from the ophiolites are of three types: (1) lead from the ophiolitic rocks and arc tonalites of the northwestern Shield and ophiolitic rocks of the Nabitah suture is similar to lead in modern mid-ocean ridge basalt, (2) anomalous radiogenic data from the Thurwah ophiolite are from rocks that contain zircons from pre-late Proterozoic continental crust, and (3) feldspar from the Urd ophiolite shows retarded uranogenic lead growth and is related either to an anomalous and perhaps primitive oceanic mantle source, or in an unknown manner to ancient continental mantle or lower crust of the eastern Arabian Shield.