Late Cretaceous subduction initiation and Palaeocene–Eocene slab breakoff magmatism in South-Central Anatolia,Turkey     

《International Geology Review》2012,54(1):66-87

The Late Cretaceous Alihoca ophiolite in the Inner Tauride suture zone (ITSZ) of South-Central Turkey represents part of a single ophiolitic thrust sheet that originated from the Inner Tauride ocean. The ophiolite contains upper mantle peridotites, cumulate wehrlites, layered-to-isotropic gabbros, and microgabbroic-to-doleritic dikes. An ophiolitic mélange beneath the Alihoca ophiolite includes blocks of limestone, peridotite, dolerite, basalt, and deep-sea sedimentary rocks (radiolarite, chert) in a matrix comprising sheared serpentinite and mudstone. Isotropic gabbro and dolerite dike rocks show enrichment in Sr, K, Rb, Ba, and Th (LILE) and depletion of Ta, Nb, Zr, Ti, and Y (HFSE), indicating an island arc tholeiite (IAT) affinity. Relatively younger dolerite rocks display low TiO₂ (<0.5 wt.%) contents, concave REE profiles with low HREE concentrations, and high LREE values, typical of boninitic affinities. The Alihoca ophiolite, hence, displays an IAT to boninitic geochemical progression in its magmatic evolution, reminiscent of many other Tethyan ophiolites in the region. It represents the remnant of a forearc oceanic crust, which developed during the early stages of subduction within the Inner Tauride ocean. Volcanic, volcano-sedimentary, and sedimentary rocks of the Uluk??la–Çamard? basin north of the ITSZ disconformably overlie the mafic-ultramafic rocks of the Alihoca ophiolite. Pillowed and massive lavas of the latest Cretaceous–Palaeocene Uluk??la Formation have alkaline basalt-to-basaltic andesite compositions, displaying relatively enriched LILE and LREE patterns with negative Nb and Ta anomalies. These geochemical features suggest that magmas of the Uluk??la–Çamard? volcanic rocks formed from partial melting of a metasomatized lithospheric mantle. This melting event was triggered by the influx of asthenospheric heat through a slab breakoff-induced window in the downgoing Tethyan oceanic lithosphere.  相似文献   

Porphyry copper deposits: strike–slip faulting and throttling cupolas     

《International Geology Review》2012,54(1):43-65

Porphyry copper deposits sometimes form during the solidification of stocks of relatively oxidized magma of intermediate composition. Most workers have assumed ore-forming systems have special chemical attributes, but none has been found that is useful to guide exploration efforts. Stocks can form where strike–slip movements generate pull-apart pathways into which intrusions can rise from batholithic magma chambers. Upwelling of buoyant, bubble-bearing magma along the sides of a stock brings magmatic fluid to shallow depths where large bubbles can separate and pool under the cupola separating solidified igneous rock from mobile magma. Where rapid seismogenic movement on the bounding strike–slip fault ruptures the solidified, but hot and ductile carapace, downward propagating extension fractures can drain an accumulation of magmatic fluid. Decompression and cooling of fluid that jets upward into extension fractures causes mineral precipitation. Where strike–slip movements cause pull-aparts to dilate with sufficient recurrences – from decades to perhaps a century or so, throttling of the fluid accumulation acts as a safety valve that prevents explosive detonation of the system. Concurrently, the upward infiltration of magmatic fluid from the cupola is strongly focused into the pull-apart and generates the characteristic concentric alteration zones that guide exploration drilling. We conclude that porphyry copper ore deposits form where strike–slip movements are concurrent with the early stages of deep-seated bubbling (?6 km) along the walls of a rapidly cooling stock of magma. Supergiant deposits form where the bubbling front extends into the top of a parent batholith.  相似文献   

Petrogenesis and evolution of late Mesozoic granitic magmatism in the Hohhot metamorphic core complex,Daqing Shan,North China     

《International Geology Review》2012,54(16):1885-1905

Late Mesozoic granitoid plutons of four distinct ages intrude the lower plate of the Hohhot metamorphic core complex along the northern margin of the North China craton. The plutons belong to two main groups: (1) Group I, deformed granitoids (148 and 140 Ma subgroups) with high Sr, LREE, and Na₂O, low Y and Yb contents, high Sr/Y and La/Yb ratios, weak or no Eu anomalies, low Rb/Ba ratios, similar initial ⁸⁷Sr/⁸⁶Sr values (0.7064–0.7071) and low Mg^# (<37 mostly, 100?×?molar MgO/MgO + FeO _t ); (2) Group II, non-deformed granitoids (132 and 114 Ma subgroups) with low Sr, relatively low Na₂O, high Y and Yb contents, pronounced negative Eu anomalies, high Rb/Ba ratios, and initial ⁸⁷Sr/⁸⁶Sr values (0.7098–0.7161). The two groups share geochemical similarities in ?_Nd(t) (–11.3 to –15.4) and T _DM2 ages (1.85–2.18 thousand million years) as well as Hf isotopic ratios in zircons. Geochemical modelling (using the MELTS code) suggests that similar sources but different depths of magma generation produced the early, high-pressure low-Mg adakitic granitoids and late, low-pressure granitoids with A-type characteristics. The early granitoids likely represent a partially melted, deep-seated, thickened lower continental crust that involved a minor contribution from young materials, whereas the later group partially melted at shallower depths. This granitic magmatic evolution coincided with the tectonic transition from crustal contraction to extension.  相似文献   

Links between Mesozoic intermediate–felsic igneous rocks and lower crustal granulite xenoliths,North China Craton: O and Pb isotopic evidence     

《International Geology Review》2012,54(4):442-452

We present and compare whole-rock and zircon O and Pb isotopic compositions for the Hannuoba granulite xenoliths and Mesozoic intermediate-to-felsic igneous rocks from the Zhangjiakou region, northern margin of the North China Craton, northeast China. The xenoliths have an overall Pb isotopic range similar to rocks from the regionally exposed Neoarchaean granulite terrain. Mesozoic zircons from different types of granulite xenoliths have a narrow range of δ¹⁸O values (6.0–7.7‰) higher than normal mantle δ¹⁸O values (～5.7‰). Mesozoic intermediate–felsic igneous rocks have O and Pb isotopic compositions indistinguishable from the Hannuoba intermediate–mafic granulite xenoliths. Our new data suggest that the Mesozoic igneous rocks and granulite xenoliths are genetically linked and that both were derived from the late Neoarchaean lower crust. This argues against previous proposals that the granulite xenoliths are either products of Mesozoic basaltic underplating or formed by mixing between mantle-derived and pre-existing crustal magmas.  相似文献   

Geochemistry of Eclogites and Metapelites from Trescolmen,Central Alps,as Observed from Major and Trace Elements and Oxygen Isotopes     

《International Geology Review》2012,54(2):95-119

Eclogites are often the only tangible high-pressure evidence we have from a paleosubduction zone, and they potentially preserve important geochemical information from the descending slab. Selected Group B/C eclogites and metapelites from the Trescolmen locality in the Adula nappe in the central Swiss Alps were chosen for a detailed investigation to determine oxygen isotope ratios and major- and trace-element compositions of the main rock-forming minerals. Complex major-element zonation patterns in garnet porphyroblasts indicate a pre-Alpine, medium-pressure growth history coupled with a high-pressure modification during the Alpine orogeny. Garnet REE patterns are notably HREE depleted in rim regions, with high overall REE content, particularly in the cores of grains. Omphacites are relatively homogenous in major elements, and show LREE- and HREE-depleted patterns, but with overall abundances of REEs lower than in garnets. These patterns are best explained by partitioning of the HREEs into garnet and the LREEs into zoisite. Oxygen-isotope systematics indicate limited fluid flow in eclogites and surrounding metapelites. δ¹⁸O values of quartz and garnet at the interface between eclogites and metapelites are indistinguishable and point to fluid exchange. Oxygen equilibrium conditions among rock-forming minerals, particularly between quartz and garnet in eclogites and metapelites, were reached, and fractionation indicates temperatures of ～600°C. The δ¹⁸O of unaltered eclogites (5.5 to 7.5 ‰) suggests a basaltic, MORB-type protolith.  相似文献   

Microthermometric and O‐ and H‐isotope characteristics of the mineralizing fluid in the Akgüney copper–lead–zinc deposit,NE Turkey     

《International Geology Review》2012,54(5):375-387

We use updated rotations within the Pacific-Antarctica-Africa-North America plate circuit to calculate Pacific-North America plate reconstructions for times since chron 13 (33 Ma). The direction of motion of the Pacific plate relative to stable North America was fairly steady between chrons 13 and 4, and then changed and moved in a more northerly direction from chron 4 to the present (8 Ma to the present). No Pliocene changes in Pacific-North America plate motion are resolvable in these data, suggesting that Pliocene changes in deformation style along the boundary were not driven by changes in plate motion. However, the chron 4 change in Pacific-North America plate motion appears to correlate very closely to a change in direction of extension documented between the Sierra Nevada and the Colorado Plateau. Our best solution for the displacement with respect to stable North America of a point on the Pacific plate that is now near the Mendocino triple junction is that from 30 to 12 Ma the point was displaced along an azimuth of ～N60°W at rate of ～33 mm/yr; from 12 Ma to about 8 Ma the azimuth of displacement was about the same as previously, but the rate was faster (～52 mm/yr); and since 8 Ma the point was displaced along an azimuth of N37°W at a rate of ～52 mm/yr.

We compare plate-circuit reconstructions of the edge of the Pacific plate to continental deformation reconstructions of North American tectonic elements across the Basin and Range province and elsewhere in order to evaluate the relationship of this deformation to the plate motions. The oceanic displacements correspond remarkably well to the continental reconstructions where deformations of the latter have been quantified along a path across the Colorado Plateau and central California. They also supply strong constraints for the deformation budgets of regions to the north and south, in Cascadia and northern Mexico, respectively.

We examine slab-window formation and evolution in a detailed re-analysis of the spreading geometry of the post-Farallon microplates, from 28 to 19 Ma. Development of the slab window seems linked to early Miocene volcanism and deformation in the Mojave Desert, although detailed correlations await clarification of early Miocene reconstructions of the Tehachapi Mountains. We then trace the post-20 Ma motion of the Mendocino slab window edge beneath the Sierran-Great Valley block and find that it drifted steadily north, then stalled just north of Sutter Buttes at ～4 Ma.  相似文献   

Water as a Petrologic Catalyst Driving 18O/16O Homogenization and Anatexis of the Middle Crust in the Metamorphic Core Complexes of British Columbia     

《International Geology Review》2012,54(2):97-130

In the Thor-Odin and Valhalla metamorphic core complexes, we have documented a remarkable uniformity of mineral δ¹⁸O values in the middle continental crust beneath the detachment faults. For example, in the Thor-Odin Complex, throughout an 8 km thick section of metasedimentary rocks and early Tertiary leucogranites in the hanging wall of the Monashee decollement (MD), quartz δ¹⁸O = 12.3 ± 0.5% (lσ S.D.) for metapelite (N = 11), 12.0 ± 0.1% for quartzite (N = 2), 12.6 ± 0.6% (N = 4) for < 1 m thick amphibolite layers, and 12.1 ± 0.4% (N = 24) for the concordant leucogranites. No exceptions have been found to this remarkable ¹⁸O/¹⁶O homogeneity except locally in a couple of thick amphibolites and within a ductile, relatively impermeable, marble-rich section. Similar zones of ¹⁸O/¹⁶O homogeneity associated with leucogranite genesis are observed throughout the mid-crustal section of the Valhalla Complex and just beneath the MD in the Monashee Complex, the only difference being that those rocks are overall 0.5 to 1.5% lower in δ¹⁸O than in the middle crust at Thor-Odin. These zones of pervasive homogenization in ¹⁸O/^l6O must be a result of exchange with magmatic or metamorphic H₂O, and these same volatiles appear to have been responsible for the leucogranite anatexis. A wide range in quartz δ¹⁸O from +8 to +16 within and below the MD suggests that this major thrust fault was impermeable to aqueous fluid flow during the partial melting stage; at that time, the basement appears to have been isolated from the mid-crustal metamorphichydrothermal system.

LITHOPROBE crustal seismic profiles establish the MD as a W-dipping, crustal-scale ramp with 20 km of vertical relief, and Carr (1992) proposed an anatectic origin for the leucogranites during decompression melting associated with tectonic shortening as the mid-crustal section moved up this thrust ramp. Partial melting of metapelites and feldspathic grits from the Late Precambrian Windermere Supergroup began in response to influx of metamorphic H₂O, aided by internal muscovite dehydration at ?8 kbar and ?750°C at the base of the Monashee ramp. Metapelites are volatile rich, but feldspar poor, whereas the opposite is true for the grit lithologies. Thus, at the base of the Monashee ramp large-scale (?30°) partial melting of the metapelites produced magmas near H₂O saturation (10 tol4 wt°), whereas the intercalated arkosic grit-derived magmas were undersaturated (5 to 6 wt°). As these H₂O-rich, pelite-derived leucogranite melts moved upward to shallower depths, they cooled adiabatically and underwent decompressive exsolution of H₂O. The released H₂O was then able to exchange oxygen with lithologies infertile to melting as it concurrently migrated through the section toward the feldspathic grit layers, where it could act as a catalyst and be re-used, promoting further hydrothermal melting of the arkosic grits. Continued decompression melting and exsolution occurred simultaneously in different parts of the section during uplift, tectonic shortening, and buoyant uprise of the magma bodies, until final crystallization of all of the leucogranites took place much higher in the crust, where almost all of the H₂O was released and again re-used for a final episode of ¹⁸O/^l6O exchange with the unmelted metamorphic lithologies. In addition to the direct ^l8O/¹⁶O exchange that takes place between the metamorphic rocks and the migrating leucogranite magmas, this use and re-use of the same H₂O during repeated episodes of partial melting and exsolution in different parts of the section seems adequate to explain the pervasive oxygen isotopic homogenization of these metasedimentary rocks. It is estimated that 25 to 30° partial melting of a typical section of the Windermere Supergroup occurred as a result of these cumulative processes, and this probably played a pivotal role in determining the susceptibility of this orogen to subsequent extensional collapse along the detachment faults.  相似文献   

Petrology and Sr–Nd characteristics of the Nova Lacerda dike swarm,SW Amazonian Craton: new insights regarding its subcontinental mantle source and Mesoproterozoic geodynamics     

《International Geology Review》2012,54(2):165-182

The NNW-trending Nova Lacerda tholeiitic dike swarm in Mato Grosso State, Central Brazil, intrudes the Nova Lacerda granite (1.46 Ga) and the Jauru granite-greenstone terrain (ca. 1.79–1.77 Ga). The swarm comprises diabases I and II and amphibolites emplaced at ca. 1.38 Ga. Geochemical data indicate that these are evolved tholeiites characterized by high LILE/HSFE and LREE/HSFE ratios. Isotopic modelling yields positive ?_Nd(T) values (+0.86 to?+2.65), whereas values for ?_Sr(T) range from positive to negative (+1.96 to -5.56). Crustal contamination did not play a significant petrogenetic role, as indicated by a comparison of isotopic data (Sr–Nd) from both dikes and country rocks, and by the relationship between isotopic and geochemical parameters (SiO₂, K₂O, Rb/Sr, and La/Yb) of the dikes. We attribute the origin of these tholeiites to fractional crystallization of evolved melts derived from a heterogeneous mantle source. Comparison of the geochemical and isotopic data of the studied swarm and other tholeiitic Mesoproterozoic mafic intrusions of the SW Amazonian Craton – the Serra da Providência, Colorado, and Nova Brasilândia bimodal suites – indicates that parental melts of the Nova Lacerda swarm were derived from the most enriched mantle source. This enrichment was probably caused by the stronger influence of the EMI component on the DMM end-member. These data, coupled with trace element bulk-rock geochemistry of the country rocks, and comparisons with the Colorado Complex of similar age, suggest a continental-margin arc setting for the emplacement of the Nova Lacerda dikes.  相似文献   

Petrology and isotope geology of mafic to ultramafic metavolcanic rocks of the Brusque Metamorphic Complex,southern Brazil     

《International Geology Review》2012,54(6):686-713

The Brusque Metamorphic Complex (BMC) is one of the main units of the Tijucas Terrain within the Dom Feliciano Belt, located in the state of Santa Catarina in southern Brazil. In the Itapema region, the BMC is composed chiefly of metasediments, including subordinate metabasalts, meta-ultramafic rocks, and clinoamphibole schists. The metavolcanic rocks form 4 m-thick lenses interlayered with metapelites and calc-silicate schists. Based on the observed textures and the associated structural, bulk-rock geochemical, and mineral chemical data, these metamafites and ultramafites were ancient lava flows of tholeiitic basalts and ultramafic cumulates. The mineral parageneses of the metabasalts are albite?+?actinolite?+?chlorite?+?epidote?+?titanite?+?magnetite and oligoclase?+?hornblende?+?epidote?+?titanite?+?magnetite, indicating progressive transformations produced under greenschist to amphibolite facies conditions. Volcanogenic metasediments show the same geochemical patterns as the metabasalts, whereas the metamorphosed ultramafic rocks consist of cumulates generated by crystal fractionation and flow segregation. The studied rocks show similar rare-earth element (REE) patterns, characterized by clearly higher normalized contents of light REEs compared with heavy REEs, without Eu anomalies in the metabasalts and positive Eu anomalies in meta-ultramafic rocks and volcanogenic metasediments. In accordance with the trace element contents that indicate a within-plate nature, the corresponding mafic melts apparently formed in the mantle by partial fusion and were subsequently enriched with crustal components during ascent into the sialic crust. The analysed ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios lie between 0.5123 and 0.5126 and 0.7067 and 0.7086, respectively, and are thus typical of tholeiitic basalts of the continental plateau type. Initial ?Nd₍₉₃₆₎ values and derived model ages (T _DM) between 1028 and 1762 million years support a mantle source or sources, with extraction and emplacement in the Neoproterozoic. Field relations and geochemical data (including isotopic data) indicate the generation of the studied mafic and ultramafic rocks in a continental rift. In the regional geologic context, the formation of the BMC volcanic and metasedimentary units marks a period of fragmentation of the Palaeoproterozoic continental crust. This extensional event is preserved regionally in gneisses from the Santa Catarina Granulitic Complex and the Camboriú Complex.  相似文献   

Initiation of slip along the Xianshuihe fault zone,eastern Tibet,constrained by K/Ar and fission-track ages     

《International Geology Review》2012,54(12):1121-1131

The Xianshuihe fault zone (XFZ) forms a boundary that accommodates crustal movement eastwards from central Tibet. The lack of well-defined time constraints has hampered the reconstruction of the geometric and kinematic evolution of the fault zone, and inhibited the development of a consistent regional tectonic model. New geochronological investigations of mica K/Ar and apatite fission-track ages on the Ganzi–Yushu segment of the XFZ indicate that fault activity started ca. 13 Ma before present, which considerably precedes the 5 Ma initiation of offset on the Xiaojiang fault (XjF) segment. Different initiation times for different fault segments clearly demonstrate that the geometric and kinematic evolution of the XFZ can be divided into two periods. The XFZ cut through the Dangjiang, Ganzi, and Gongga Shan areas, and reached the Qinghe–Yanyuan area during an early stage, from ca. 13 to 5 Ma, as a boundary fault with lateral mass transfer from the interior to southeast Tibet. At the southern edge of the XFZ, the movement took advantage of the southern segment of the Longmen Shan thrust fault – the Jinhe–Qinghe thrust fault. During the late stage, from 5 Ma to present, the fault zone passed through the Yushu, Ganzi, and Gongga Shan areas, then offset the Longmen Shan thrust fault belt and reached the Kunming area. The Jinhe–Qinghe fault was abandoned in the later period as the southern part of the XFZ, and the XjF became active because of continuous clockwise rotation of the crust around the eastern syntaxis.  相似文献