Tonalite Plutons Research Articles

Ancient oceanic crust in island arc lower crust: evidence from oxygen isotopes in zircons from the Tanzawa Tonalitic Pluton.

Ancient oceanic crust in island arc lower crust: evidence from oxygen isotopes in zircons from the Tanzawa Tonalitic Pluton.

Zircon U–Pb dating from the mafic enclaves in the Tanzawa Tonalitic Pluton, Japan: Implications for arc history and formation age of the lower-crust

The petro-chemical characteristics of the arc lower-crust, important for understanding continental growth, have been rarely obtained because of their scarcity at the surface of the Earth. To constrain the formation age of the arc lower-crust, U–Pb zircon dating was applied to mafic enclaves in tonalites of the Tanzawa Tonalitic Pluton (TTP), which is regarded as the exposed middle crust of the former Izu–Bonin–Mariana arc, using a laser ablation-inductively coupled plasma–mass spectrometer (LA-ICP-MS).The texture and shape of mafic enclaves indicate an injection of mafic magma into tonalitic magma at the mid-crustal level. While 44 zircon grains from a host tonalite show a narrow-age distribution with a mean age of 4.6±0.2Ma, 301 zircon grains from 9 mafic enclaves show wide-age distributions from ca. 5 to 43Ma. This study is the first to reveal a U–Pb age older than previously reported for the rock materials that compose the TTP, now identified to be 18Ma compared with an age range from 4Ma to 9Ma. Because there are no other components of the TTP yet identified to be older than 17Ma, the zircons separated from the TTP in this study which dated to be 18–43Ma are interpreted to be xenocrysts derived from the arc lower-crust beneath the TTP. The oldest zircon age obtained from the mafic enclaves indicates that the formation of the arc lower-crust beneath TTP took place before 42.9±8.6Ma, consistent with the history of the Izu–Bonin–Mariana arc. This is the first time that the formation age of the lower crust has been estimated using zircons from mafic enclaves. This study shows that the zircon U–Pb dating from mafic enclaves in granites can yield significant information about the age of the continental lower crust.

Origin of ~2.5 Ga potassic granite from the Nellore Schist Belt, SE India: textural, cathodoluminescence, and SHRIMP U–Pb data

In a geochemical and geochronological investigation of Archean and Proterozoic magmatism in the Nellore Schist Belt, we conducted SHRIMP U–Pb analyses of zircons from two cospatial granitic bodies at Guramkonda and Vendodu. The former is a Ba- and Sr-rich hornblende-bearing tonalite, whereas the latter is a Rb-, Zr-, Pb-, Th-, U-, and REE-rich biotite-bearing leucogranite. The Guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt, whereas the Vendodu leucogranite contains residual/partially melted plagioclase grains. Both rock types contain two generations of zircon: tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt (quartz + feldspar matrix), and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth. Cathodoluminescence images also clearly document two distinctly textured varieties of zircon: the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark (U-rich) and light (U-poor) regions; the leucogranite contains U-rich zoned zircons and U-poor zircon cores mantled by U-rich rims. The REE chemistry of zircon cores from the Vendodu leucogranite is very similar to the REE of zoned zircons from the Guramkonda tonalite. Zircon ages from both plutons exhibit bimodal distributions in U–Pb concordia diagrams. The tonalite defines an age of 2,521 Ma ± 5 Ma for zoned magmatic zircons and 2,485 Ma ± 5 Ma for unzoned newlyprecipitated zircons, whereas the leucogranite has an age of 2,518 Ma ± 5 Ma for U-poor zircon cores (relics of the tonalite pluton) and 2,483 Ma ± 3 Ma for U-rich zoned magmatic zircons. The trace element geochemistry of the ~2,520 Ma zircons is distinctly different from the ~2,485 Ma zircons, irrespective of the host rock. Our textural, CL image, and SHRIMP U–Pb analyses document the origin of the leucogranite by partial melting of the tonalite. High alkalis (Na2O + K2O), Rb, Nb, HREE, FeOt/MgO and low Ca, Al, Ba, Sr, and large negative Eu anomalies characterize the leucogranite as a thermal minimum melt, whereas the very low K and Rb of the tonalite attests to its residual nature. We suggest that the leucogranite formed by high-T (900–950°C), moderate-pressure (<10 kbar) dehydration partial melting of the tonalite under reducing conditions. The calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction-zone environment. The leucogranite probably formed in a post-collisional realm immediately after accretion of the tonalitic crust.

Collisional accretion of a Late Ordovician oceanic island arc, northern Tasman Orogenic Zone, Australia

A distinctive Late Ordovician volcano-sedimentary terrane, embracing the Carriers Well Formation and Everetts Creek Volcanics and dismembered slivers now structurally intercalated in the adjoining Wairuna Formation, is located within the Broken River Province of the northern Tasmanides. It abuts a basement of mafic–ultramafic rocks (Gray Creek Complex) and overlying Early Ordovician deep marine sedimentary and volcanic strata (Judea Formation) which host tonalitic plutons. The terrane lies at the western, inboard-margin of the Camel Creek Subprovince, a broad tract of multiply deformed mid-Paleozoic turbidites with minor basalt and chert variously interpreted as the infill of a backarc basin or an accretionary wedge. U–Pb dates from detrital zircon indicate a maximum Late Silurian age for siliciclastic rocks from the previously undated Wairuna Formation. Geochemistry of volcanic rocks from the volcano-sedimentary terrane show them to be largely of mafic to intermediate compositions of calc-alkaline affinity, comparable with broadly coeval Macquarie Arc volcanic suites of the southern Tasmanides. Trace-element systematics identify a subduction relationship for the volcanic suite and V/Ti employed as a discrimination tool identifies the terrane as representing an oceanic island arc, consistent with its sedimentary facies which include volcaniclastic mass flow deposits, limestone, and radiolarian chert. Continent-derived sandstone in the sedimentary assemblage, confirmed by the ages of detrital zircon from a sandstone sample from the Carriers Well Formation, indicates that the oceanic island arc developed proximal to the Late Ordovician continental margin of East Gondwana. Its nature and location bear on the tectonic setting of the entire Camel Creek Subprovince, for which interpretation as an Early Silurian–Early Devonian accretionary wedge is favoured. Collision of the island arc with the continental margin, and associated deformation of part the intervening oceanic crustal tract, now represented by the Gray Creek Complex and its sedimentary cover (Judea Formation) registers the initiation of subduction accretion in late Early Silurian (Llandoverian) time. It marks early-stage orogenesis in the Broken River Province, accurately timed by stratigraphic relationships in the basinal succession developed in the Graveyard Creek Subprovince immediately to the west of the arc assemblage. Tectonism was regionally developed in north Queensland at this time, coeval with the Benambran Orogeny of the Lachlan Orogen in which the Macquarie Arc was likewise accreted to the East Gondwana margin. Benambran orogenesis marks a general phase of shortening, and removal by subduction, of oceanic crust and inversion of continent-derived overlying sedimentary cover along the East Gondwana margin.

The Rio Capim volcanic–plutonic–sedimentary belt, São Francisco Craton, Brazil: Geological, geochemical and isotopic evidence for oceanic arc accretion during Palaeoproterozoic continental collision

The collision of oceanic arcs with continents is a common plate tectonic process in the Phanerozoic, but its recognition in the Precambrian is hampered by deformation and metamorphism. The Rio Capim volcanic–plutonic–sedimentary belt lies in sharp tectonic contact with Archaean rocks of the Uauá block in the northern part of the São Francisco craton. Field relationships and high-precision geochronology indicated that the Rio Capim basalts, gabbros, diorites, and dacites were emplaced approximately at 2148–2143 Ma, and later intruded by 2128 Ma-old diorite to tonalite plutons. All rocks were metamorphosed under amphibolite to granulite facies conditions mainly between 2080 Ma and 2070 Ma, but deformation may have lasted until about 2040 Ma as estimated from syn-deformation zircon and titanite grains. The association of basalt, andesite, dacite, and their plutonic counterparts, combined with their positive ε Nd( t) values and incompatible trace element geochemical signatures similar to island arc magmas, support the proposition that the Rio Capim belt was a Palaeoproteorozoic intra-oceanic arc sequence that collided with a continent, of which the Mesoarchaean Uauá block is a remnant. The implications for the regional evolution and metallogenesis are also discussed.

Structure, emplacement, and tectonic setting of Late Devonian granitoid plutons in the Teplá–Barrandian unit, Bohemian Massif

The Stěnovice and Cista granodiorite–tonalite plutons are small (~27 and ~38 km2, respectively) intrusions that are largely discordant to regional ductile structures in the center of the upper-crustal Tepla–Barrandian unit, Bohemian Massif. Their whole-rock and trace-element compositions are consistent with medium-K calc-alkaline magma, generated above a subducted slab in a continental margin arc setting. The U–Pb zircon age of the Stěnovice pluton, newly determined at 375 ± 2 Ma using the laser ablation ICP-MS technique, is within the error of the previously published Pb–Pb age of 373 ± 1 Ma for the Cista pluton. The two plutons also share other characteristics that are typical of concentrically expanded plutons (CEPs), such as elliptical cross-section in plan view, steep contacts, inferred downward-narrowing conical shape, faint normal zoning, and margin-parallel magmatic foliation decoupled from the regional host-rock structures. We interpret the Stěnovice and Cista plutons as representing the initial Late Devonian stage of much more voluminous early Carboniferous arc-related plutonism (represented most typically by the Central Bohemian Plutonic Complex) in the upper crust of the central Bohemian Massif. These two plutons are important tectonic elements in that they indicate an overall shift of the arc-related plutonic activity from the ~NW to the ~SE, accompanied with a general compositional trend of the magmas from medium-K calc-alkaline to shoshonitic/ultrapotassic. Such a pattern is compatible with SE-directed subduction of the Saxothuringian Ocean beneath the Tepla–Barrandian overriding plate as a cause of arc-related magmatism in this part of the Bohemian Massif.

Significance of “gneissic” rocks in the Liscomb Complex, Nova Scotia

The Liscomb Complex comprises Late Devonian intrusive rocks (principally peraluminous granite) and medium- to high-grade metamorphic rocks (“gneisses”) that collectively are hosted by low-grade (greenschist facies) metasediments of the Cambro-Ordovician Meguma Group. The conventional view that these “gneisses” contain high-grade mineral assemblages and represent basement rocks has recently been challenged, and indeed, some of the rocks previously mapped as gneisses, particularly metapelites, have isotopic compositions resembling the Meguma Group. Amphibole-bearing enclaves in the Liscomb plutons, however, are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the Popes Harbour lamprophyre dyke, south of the Liscomb area. Metasedimentary enclaves with Meguma isotopic signatures can contain garnets with unzoned cores (implying high temperatures) that host high-grade minerals (prismatic sillimanite, spinel, and (or) corundum) and are enclosed by retrograde-zoned rims. These features are interpreted here as having formed during and following the attainment of peak temperatures related to Liscomb magmatism. The amphibole-bearing meta-igneous rocks described here contain cummingtonite or hornblendic amphibole and occur as enclaves in granodioritic to tonalitic plutons. They are mineralogically, texturally, and isotopically distinct from Meguma metasediments and at least some of the plutonic rocks that enclose them, so remain the most likely candidate for basement rocks in the Liscomb Complex.

Evidence of a mantle contribution in the genesis of magmatic rocks from the Neogene Batu Hijau district in the Sunda Arc, South Western Sumbawa, Indonesia

Whole-rock geochemical and radiogenic data are combined with in situ trace and isotopic analyses of amphibole grains to characterize the source and the emplacement mechanisms of the magmas of the Sunda arc in the Batu Hijau district, Sumbawa, Indonesia. The low-K calc-alkaline magmatic suite in the area is characterized by a distinctively juvenile signature (143Nd/144Nd ~0.5130). Whole-rock trace element and Pb isotopic data (207Pb/204Pb ~15.603) suggest the involvement of a minimal (<0.1%) sediment component in arc petrogenesis. During the petrogenesis of the calc-alkaline plutons, the involvement of fluids that were not entirely derived from the dehydration of a subducting slab is reflected in the mineral chemistry of the primary hydrous magmatic amphiboles, which contain very low B and Li concentrations. We argue that the B- and Li-poor fluids implicated in the petrogenesis of the calc-alkaline melts were at least partially derived from dehydration of uprising asthenospheric mantle. The δD values of selected hydrous magmatic amphibole grains range between ca. −70‰ and 0‰, consistent with an original mantle-derived signature, which was subsequently modified due to a de-hydrogenation process. We put forward the hypothesis that in the Batu Hijau district an arc-transverse fault system facilitated the rise of asthenosphere-derived melts above a kink, or tear, in the subducting Indian Ocean Plate that underlies the Sunda arc. The melts ascended to upper-crustal levels and underwent fractionation while interacting with the arc crust or metasomatized lithospheric mantle wedge. As a result of this study, we emphasize the significance of crustal-scale faults as conduits that connect the mantle to upper-crustal levels in arc settings. The de-hydrogenation process that the tonalite plutons underwent in the Batu Hijau district may have been crucial to the genesis of associated Cu–Au porphyry mineralization and the development of the Pliocene Batu Hijau deposit. Consequently, we argue that deep structures may facilitate the efficient release of mineralizing fluids at high crustal-levels.

High-pressure thermal aureoles around two Neoproterozoic synorogenic magmatic epidote-bearing granitoids, Northeastern Brazil

Unusual high-pressure inner thermal aureoles are described from the Minador and Angico Torto epidote-bearing tonalitic plutons that emplaced into greenschist-facies metasedimentary rocks of the Neoproterozoic Cachoeirinha-Salgueiro belt, northeastern Brazil. The foliated pelitic hornfelses display the mineral assemblage garnet, kyanite, staurolite, muscovite, biotite, plagioclase ± quartz. Rare fibrolite is only found very close to the contacts. Hornfelses display steep mineral lineations and steeply-dipping foliations concordant with magmatic contacts. Leucocratic veinlets containing quartz, oligoclase, garnet, kyanite, staurolite, rutile and ilmenite suggest that limited melting conditions were reached very close to magmatic contacts ( T ⩾ 650 °C, P around 8 kbar). These high-pressure hornfelses form a few meters thick, rigid envelopes around the two plutons. Contrary to known examples of kyanite-bearing hornfelses that recorded high-temperature decompression, the nearly isobaric cooling down to ca. 450 °C is constrained by 3.20–3.30 Si contents of retrogressive phengites from both inner hornfelses and ductilely-deformed tonalite at the pluton margins. Isograds and bathograds are, therefore, apparently telescoped due to HP/LT shearing, possibly caused by subsequent differential vertical movements affecting these two solidified plutons. The unusual depth of emplacement of these syn-kinematic calc-alkaline plutons is explained by a tentative geodynamic model involving a pre-620 Ma-subduction setting. Resumen Las aureolas internas que rodean dos plutones tonalíticos emplazados dentro de rocas cajas en facies esquistos verdes del Cinturón-plegado Cachoeirinha-Salgueiro al noreste de Brasil, contienen hornfelses pelíticos foliados con granate, kyanita, estaurolita, muscovita, biotita, plagioclasa ± cuarzo. Fibrolita es rara ó es encontrada solamente cerca de las zonas de contacto. Los hornfelses desarrollaron foliaciones concordantes con buzamiento fuerte cerca de los contactos magmáticos y muestran lineaciones minerales casi verticales. Venillas leucocraticas contienen cuarzo, oligoclasa, granate, kyanita, estaurolita, rutilo e ilmenita sugiriendo que el límite en las condiciones de fusión fueron cercanas a los contactos magmáticos ( T ⩾ 650 °C, P entorno 8 kbar). Estos hornfelses de alta presión forman una cobertura rígida de poco metros entorno a los dos plutones. Contrariamente a lo conocido en diferentes ejemplos de hornfelses con kyanita con registro de descompresión, la isóbara de enfriamiento para cerca de 450 °C está controlada por el contenido de Si 3.20–3.30 de fengitas retrógradas, en los hornfelses internos y en las márgenes de los plutones donde las tonalitas están deformadas ductilmente. Isógradas y batógradas son, por lo tanto, aparentemente telescopadas debido a cizalla de alta presión/baja temperatura, posiblemente causado por subsecuentes movimientos verticales diferenciales afectando estos dos plutones solidificados. El emplazamiento a una profundidad inusual de estos plutones calci-alcalinos syn-cinemáticos es explicado por un modelo geodinámico tentativo que involucra un escenario de subducción pre-620 Ma.

An unusual occurrence of probable Pleistocene corestone within a Cretaceous dioritic enclave, Peninsular Ranges, California

Abstract We here document an unusual occurrence of probable Pleistocene corestone within an ∼ 5 m by ∼ 5 m dioritic enclave contained within a Cretaceous tonalitic pluton, Santa Margarita Ecological Reserve, SW California. The enclave lies within ∼ 3.5 km of the seismically active ∼ 1.5–2 Ma Elsinore fault zone, and may have been subjected to ∼ 70–90 ground shaking events since ∼ 22,000–18,000 years ago. The studied corestone is elliptical with major axis measuring ∼ 55 cm and minor axis ∼ 26 cm in length. It is surrounded by a discontinuous ∼ 7 cm thick rind that breaks apart under slightly greater finger pressure than does the surrounding saprolite. In order to assess changes in the physical and chemical properties of the corestone, rind, and saprolite we collected along an ∼ 66.5 cm long traverse 4 samples from the corestone, 1 from the rind, and 5 from the saprolite for bulk and grain density, porosity, and major and trace element analyses. Textural and clay mineral data, along with the redistribution of elemental mass, indicate that the weathering of biotite, and to much lesser degrees apatite and the An-rich cores of plagioclase, played critical roles in the production of saprolite especially in a narrow ∼ 20 cm wide zone adjacent to the rind of the corestone. Within this zone bulk densities reach their minimum values, while porosities and positive volume strains reach their maximums. In addition, the maximum loss of K, Fe, Mn, and Ca mass occurred just inside this region, and is paralleled by the highest CIA values and highest additions of Si and Rb mass. In contrast, the masses of Na and Sr are progressively increased and decreased, respectively along the entire sampling traverse while the loss of Mg, Ti, and P mass is episodic with the greatest losses occurring within the narrow zone adjacent to the rind and at the end of the traverse. The above observations indicate that the conversion of biotite to expandable mixed-layered clay minerals, aided by the alteration of the An-rich cores of plagioclase, produced sufficient stress on grain boundaries that a weakening or loss of intercrystalline cohesion occurred. However, unlike the well documented isovolumetric development of saprolite in other areas, at the study site saprolitization was accompanied by a volume expansion. We speculate that repeated ground shaking in response to earthquakes generated in the nearby Elsinore fault zone may be responsible for this difference. Important byproducts of ground shaking would include additional weakening or loss of intercrystalline cohesion, and the production and enhancement of new and older fluid pathways respectively. The fact that the most intensely altered material at the study site lies adjacent to the boundary between the rind and saprolite, suggests that this interface acted to guide fluid flow around the corestone and rind and into the adjacent saprolite where elemental mass was redistributed down various paths leading to the underlying water table. Most of the leached elemental mass was removed from the area of the sampling traverse, but small increases in Si, Rb, and Na mass suggest redistribution of these elements from elevated areas outside the area sampled during this study. The result of the above complex set of processes is a variably porous and chemically altered saprolitic enclave that is still undergoing modification as it adjusts to the vicissitudes of the paralithic environment and continuing ground shaking during earthquakes generated along the Elsinore fault.

As anomalias geológicas e geofísicas localizadas ao norte de Itapororoca (PB), folha Guarabira

Anomalies concerning the litho-stratigraphic, geomorphic and hydrogeologic contexts were detected on the region north of Itapororoca-PB. These anomalies are concentrated in the area coined as "Cretacic volcanics" by the system DNPM-CPRM (1974), in the basement of the Barreiras Group. In this region, two classic terranes of the Borborema Province were recognized: S. José do Campestre (SJC, Paleoproterozoic) and Alto Pajeú (TAP, Eoneoproterozoic, respectively to the north and to the south of the Patos Lineamment (LP). Along LP there is preserved part of the cover rocks of the SJC, phyllonitized and drastically folded, of the Seridó Group. These terranes and cover rocks are pierced by Brasiliano plutons that display very distinct petrogenetic and isotopic characteristics. The fist case of of juvenile tonalitic plutonism of the Province is there recorded. A silicious duricrust, with many textural facies is present in the area, as relicts of previous plateau, between LP and the Mamanguape River (aligned by an E-NE normal fault). Anomalies of F, Cr and Ni were detected in this duricrust, in which some sparce fractions of ultramafic and alkaline rocks could be identified. Besides the geological mapping, a gravity survey of 548 new gravity stations was performed for the purpose in addition to 452 prexisting stations of the Observatório Nacional and of the UFRN, so covering the area of the duricrust as well as the area of some circular structures, in the surroundings.

As anomalias geológicas e geofísicas localizadas ao norte de Itapororoca (PB), folha Guarabira

Anomalies concerning the litho-stratigraphic, geomorphic and hydrogeologic contexts were detected on the region north of Itapororoca-PB. These anomalies are concentrated in the area coined as “Cretacic volcanics” by the system DNPM-CPRM (1974), in the basement of the Barreiras Group. In this region, two classic terranes of the Borborema Province were recognized: S. Jose do Campestre (SJC, Paleoproterozoic) and Alto Pajeu (TAP, Eoneoproterozoic, respectively to the north and to the south of the Patos Lineamment (LP). Along LP there is preserved part of the cover rocks of the SJC, phyllonitized and drastically folded, of the Serido Group. These terranes and cover rocks are pierced by Brasiliano plutons that display very distinct petrogenetic and isotopic characteristics. The fist case of of juvenile tonalitic plutonism of the Province is there recorded. A silicious duricrust, with many textural facies is present in the area, as relicts of previous plateau, between LP and the Mamanguape River (aligned by an E-NE normal fault). Anomalies of F, Cr and Ni were detected in this duricrust, in which some sparce fractions of ultramafic and alkaline rocks could be identified. Besides the geological mapping, a gravity survey of 548 new gravity stations was performed for the purpose in addition to 452 prexisting stations of the Observatorio Nacional and of the UFRN, so covering the area of the duricrust as well as the area of some circular structures, in the surroundings.

北上山地，千厩トーナル岩体の岩石化学的特徴：特にアダカイト質岩との関連について

北上山地，千厩トーナル岩体の岩石化学的特徴：特にアダカイト質岩との関連について

Whole-rock geochemistry of some tonalite and high Mg/Fe gabbro, diorite, and granodiorite plutons (sanukitoid suites) in the Kuhmo district, eastern Finland

New whole-rock geochemical analyses (major and trace elements) of three tonalite plutons and high Mg/Fe granodiorites and diorites, termed sanukitoid suites, are presented from Neoarchaean Kuhmo district, eastern Finland. The 2.83 Ga and 2.78 Ga tonalites display only moderate fractionation of LREE/HREE thus demanding only minor or no residual garnet in their respective source regions. However, the more prominent HREE depletion in the 2.75 Ga tonalites requires more significant garnet residue. The relatively wide compo sitional variation in the tonalites implies that Archaean TTG (tonalite-trondhjemite-granodiorite) magmatism was derived in general from sources that exhibited considerable mineralogical heterogeneity. The ~2.70 – 2.74 Ga high-Mg/Fe granodiorites and diorites are characterised by strong fractionation of LREE/HREE, relatively high Ba and Sr, and elevated abundances of Cr and Ni as well systematic depletions in P and Ti, which are all typical features of Archaean sanukitoid suites. Some of the granodiorite plutons contain compositionally intermediate high-K enclaves, which share these geochemical characteristics, implying a contribution of mafic-intermediate LILE- and LREE- enriched source for the magma. The geochemical characteristics of the studied plutonic rocks demonstrate the role of contrasting sources for the tonalites and sanukitoid suites in the Kuhmo district.

Exploring the potential of allanite as a geochronometer of high-grade crustal processes

The REE-rich accessory mineral allanite plays a key role in the storage and mobility of geochemically important trace elements (LREE, Th) in magmatic and high-grade rocks. Allanite occurs in a wide range of rock types, but of particular interest is its common presence in mafic, migmatitic and high-pressure rocks. We report on the response of allanite trace element chemistry and its U-Th-Pb isotopic system to magmatism, partial melting and eclogite-facies metamorphism. In situ U, Th–Pb geochronology of allanite has been carried out using SHRIMP ion microprobe in conjunction with LA ICP-MS analysis. We analyse allanite with a variety of FeO and trace element compositions from anatectic rocks, granodioritic to tonalitic plutons and high-pressure mafic rocks. Allanite Th– Pb ages are capable of reliably addressing geochronological problems with a precision of 1–2% and 2–5% (2r) for magmatic and metamorphic rocks, respectively. This study allows preliminary conclusions to be made for the closure temperature of allanite. In the case studies presented, no indication of inheritance has been observed for allanite. The trace element composition of allanite records changes in paragenesis. Allanite HREE content relative to LREE provides an indicator for the co-crystallization of garnet. Where allanite formed in migmatites display a small negative Eu anomaly, eclogitic allanite lacks a Eu anomaly, which is related to crystallization in the absence of plagioclase. In addition, the Sr content in allanite can be used as an indicator of crystallization above the stability field of plagioclase. This study demonstrates that allanite can be correlated to major metamorphic and rock-forming minerals and therefore the P-T conditions of crystallization.

Stable Isotope Evidence for Large-Scale Infiltration of Metamorphic Fluids Generated during Shallow Subduction into the Eastern Peninsular Ranges Mylonite Zone (EPRMZ), Southern California

New δ18O and δD data confirm and amplify the conclusion by Holk et al. (1991) that, during the late Cretaceous or earliest Tertiary, there was large-scale infiltration of metamorphic water at depths of at least 15 km into the lower plate of the EPRMZ, a major crustal shear zone that transects a 15 x 15 km tonalite pluton west of Borrego Springs, California. Whole-rock δ18O values in undeformed tonalite are +11.2 ± 0.4 (n = 9) and in mylonitized tonalite are +12.3 ± 0.3 (n = 21), indicating a 1 per mil increase in 18O during open-system water-rock exchange. Quartz, feldspar, and biotite δ18O values in undeformed tonalite are +12.9 ± 0.4 (n = 8), +11.0 ± 0.6 (n = 8), and +7.7 ± 0.7 (n = 8), compared with +13.5 ± 0.2 (n = 8), +11.8 ± 0.3 (n = 9), and +9.2 ± 0.3 (n = 10) in mylonitized tonalite. The extremely uniform quartz-feldspar and quartz-biotite 18O fractionations in the mylonitized tonalite indicate equilibration at 500-600°C. Biotite and hornblende δD values from undeformed tonalite are -73 ± 3 (n = 12), compared with -52 to -68 (n = 21) in mylonitic tonalite, even though the biotite and hornblende have the same Fe/Mg (≈1.25; Anderson, 1983) in both kinds of tonalite. This implies that the EPRMZ was infiltrated by large amounts of high-D water (δD ≈ -10 to -30) at 500-600°C, probably derived at depth by dehydration of subducted oceanic crust or underplated volcanogenic sediments. Syndeformational tonalitic pegmatites have quartz and feldspar δ18O values of +13.5 ± 0.3 (n = 4) and +12.2 ± 0.3 (n = 6), and biotite and muscovite δD values of -64 ± 1 (n = 2) and -42 (n = 1), suggesting a genetic relationship between the pegmatites and these deep-seated fluids. Higher whole-rock δ18O values (+13.2 to +16.1, n = 3) and much lower δD values (-72 to -114, n = 3) occur where the EPRMZ cuts upper-plate amphibolite-facies metasediments. The shift to lower δD values in these finer grained rocks probably occurred during interactions with heated meteoric groundwaters during Pliocene detachment faulting associated with the opening of the Gulf of California.

Contrasting structural histories of the Salmon River belt and Wallowa terrane: Implications for terrane accretion in northeastern Oregon and west-central Idaho

The Salmon River belt is composed of metamorphic tectonites with poorly dated Mesozoic and Paleozoic protoliths that lie between volcanic arc assemblages of northeastern Oregon and the western margin of the North American continent in west-central Idaho. To the west, the Salmon River belt overlies late Paleozoic and Mesozoic volcanic arc rocks of the Wallowa terrane on a shallowly east-dipping fault; to the east the belt is separated from North American rocks by the western Idaho shear zone. The Salmon River belt is divided into western and eastern assemblages composed of greenschist- and amphibolit-facies metamorphic tectonites, respectively, and is deformed in three and locally four generations of penetrative structures. First-generation structures are synmetamorphic, west-vergent, tight to isoclinal folds with a penetrative axial-planar cleavage and downdip stretching lineation. Second- and third-phase structures are postmetamorphic and consist of open to close, upright to slightly overturned folds with north-south and east-west trending penetrative axial-planar cleavages, respectively. Low-angle faults truncate metamorphic structures within the Salmon River belt and juxtapose rocks with different metamorphic grades. The faults formed together with second-generation structures and are deformed by third-generation folds. The age of metamorphic and postmetamorphic structures is constrained by deformed carbonate rocks of Late Triassic age and clastic rocks of late Early Jurassic age and by the ca. 145 Ma emplacement of an undeformed tonalitic pluton. Along the eastern margin of the Salmon River belt, structures are overprinted and transposed by a younger, steeply east-dipping mylonitic shear zone, the mid-Cretaceous western Idaho shear zone. Synmetamorphic structures within the Salmon River belt formed before amalgamation of the Wallowa terrane and the North American fringing-arc complex and developed as part of a regional belt of intra-arc metamorphic tectonites. The pr-Cretaceous docking of the Wallowa terrane is recorded within the Salmon River belt by development of postmetamorphic structures and predates formation of the western Idaho shear zone. The western Idaho shear zone formed after the amalgamation of the arc terranes and records the intraplate deformation associated with the collapse of the fringing-arc and back-arc-basin system.

Neoarchaean volcanic rocks, Central Hearne supracrustal belt, Western Churchill Province, Canada: geochemical and isotopic evidence supporting intra-oceanic, supra-subduction zone extension

The Kaminak segment of the Central Hearne supracrustal belt (CHSB) Western Churchill Province, Canada, comprises a diverse sequence of Neoarchaean volcanic and less abundant metasedimentary rocks that were emplaced as two assemblages between 2695–2711 Ma and 2681–2686 Ma, respectively. These are intruded by uncommon pre-tectonic diorites and tonalites (ca. 2691 Ma), voluminous syn-tectonic tonalites and granodiorites (ca. 2679–2686 Ma) and, rare post–tectonic potassic monzogranite and syenite (ca. 2659–2666 Ma). Metasedimentary rocks include turbidites, epiclastic tuffs, minor iron formation (oxide) and rare volcanic conglomerates. Volcanic rocks of assemblage I include abundant pillowed and massive basalts to andesites with less common silicic lavas, tuffs and volcaniclastic debris flow deposits. Assemblage II contains voluminous silicic tuff and volcaniclastic debris flow deposits but fewer basaltic to andesitic flows. The critical diagnostic feature of the CHSB is the stratigraphic intercalation of compositionally diverse basaltic, andesitic and felsic volcanic rocks throughout both assemblages. Mapping, U–Pb geochronology and lithogeochemistry suggest that an initial MORB-like basaltic plain containing widespread intercalations of dacite to rhyolite was replaced at ca. 2688 Ma by a relatively short-lived, dacite to rhyolite dominated magmatic environment characterized by localized felsic volcanic centres and a bloom of 2686–2679 Ma tonalitic to granodioritic plutons. Basaltic to andesitic rocks are dominated by iron-rich tholeiites, although the proportion of calc-alkaline rocks increases with silica content. Felsic volcanic rocks all exhibit calc-alkaline affinities. The wide range in chemistry of the basaltic to andesitic rocks of both volcanic assemblages implies diverse mantle sources capable of generating voluminous MORB-, with less common ARC-, NEB(OIB)- and rare BABB-like rocks. Similarly, the variable composition of the felsic volcanic rocks indicates both anatexis of eclogitic to garnetiferous mafic crust and also extensive fractionation of mafic precursors in crustal magma chambers. Two geochemically distinct, arc-like mafic suites were generated through contamination of primary mantle-derived magmas by juvenile, ca. 2700 Ma silicic crust either in their mantle source or through assimilation upon ascent. ϵNd t isotopic data are comparable to contemporaneous depleted mantle with only local evidence for incorporation of older, ⋙2700 Ma crust. The CHSB may have formed via tectono-magmatic processes comparable to those of the Eocene, infant-arcs of the SW Pacific, whereby the formation of a thick sequence of coeval, intercalated, compositionally diverse mantle- and crustal-derived rocks, are generated in an extensional supra-subduction setting. The cessation of supra-subduction zone extension at ca. 2688 Ma, was followed by the short-lived development of felsic volcanic edifices (incipient arc), the extrusion of mafic to felsic magmas and the concomitant intrusion of voluminous syn-kinematic tonalitic plutons. This accompanied a major change in the tectono-magmatic setting accompanying and presumably following the termination of extensional, supra-subduction zone processes.

Emplacement-related microstructures in the margin of a deformed pluton: the San José tonalite, Baja California, México

Abstract In the San Jose tonalite pluton, Baja California, Mexico, interior magmatic fabrics give way progressively to subparallel, solid-state fabrics with S–C structures near the margins. The outer rim (‘marginal northern unit’), approximately 1 km thick, is inferred to have been deformed during the emplacement of the inner parts of the magma chamber. The rocks show a transition from strongly deformed at the outer edge to progressively less deformed towards the inner edge of the marginal northern unit. A small amount of melt appears to have been present during the deformation, as indicated by (1) apparent contact melting between some grains of plagioclase, and (2) coarse-grained aggregates and single-grains of quartz, K-feldspar and/or more sodic plagioclase in fracture-controlled openings in primary plagioclase crystals. The more sodic plagioclase typically grew as rims on the walls of the openings in twin continuity with the primary plagioclase. The deformed rocks have a prominent foliation marked mainly by aggregates of biotite, with contributions by hornblende and titanite grains, as well as apparently recrystallized aggregates of quartz and/or feldspar, at least some of which may have originated as zones of fragmentation. Many primary grains of plagioclase, hornblende, biotite and titanite show evidence of brittle deformation and fragmentation. The shapes of aggregates in the folia, relationships between incipient folia and residual primary grains, and occurrences of many folia in narrow zones transecting strong minerals (plagioclase and hornblende), suggest that brittle processes may have controlled at least some of the foliation development. However, evidence of strain-induced grain-boundary migration and bulge nucleation of new grains of quartz and plagioclase indicates that dislocation creep also played a part in the deformation. Recrystallization of plagioclase, together with intrusion of the pluton at greenschist facies ambient temperatures, confirms high-temperature deformation, with heat derived from the cooling magma.

Foliation development and progressive strain-rate partitioning in the crystallizing carapace of a tonalite pluton: microstructural evidence and numerical modeling

The San José pluton in Baja California, México, comprises at least two well-defined, texturally distinct units. The northern unit was intruded by the central unit after the former had extensively crystallized at its margins. During intrusion of the central unit, the margin of the northern unit underwent brittle and crystal–plastic deformation, at least part of which occurred in the presence of residual melt. We infer that biotite grains in this rock readily deformed by slip and frictional sliding along (001) planes, which caused strain-rate and differential-stress gradients across their grain boundaries into the surrounding plagioclase framework causing it to fracture. These microfractures grew and coalesced, and became sites of localized ductile flow. Continued development of these microshear zones led to coalescence of biotite grains, mainly by mechanical entrainment, and ultimately to a pervasive mylonitic foliation. Thus, in a single deformation, these rocks passed through a brittle–ductile transition. The development of an anastomosing network of ductile microshear zones allowed the progressive partitioning of strain rates, probably over several orders of magnitude, between the microshear zones and intervening polymineralic aggregates. Numerical experiments were conducted to evaluate the process of biotite-assisted fracturing of the stress-supporting framework, and the progressive evolution of differential stress and strain rate. The results are consistent with experimental evidence that biotite is extremely weak in shear, and that phyllosilicate-bearing rocks may accommodate strain rates several orders of magnitude higher than the bulk strain rate. This study also supports previous suggestions that strain rates associated with the growth of crustal magma chambers may be several, to many, orders of magnitude greater than those normally associated with regional tectonic deformation.