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We present a new U-Pb zircon age and geochemical data from an intermediate calc-alkaline pluton with adakitic affinity from NW Hispaniola (Haiti). The data provide important constraints and offer new insight on the Late Cretaceous (~90 Ma) development and geological evolution of the Greater Antilles Arc (GAA). The pluton intrudes Cretaceous basalts and mafic basic tuffs of the volcanic arc domain in the Massif du Nord (northern Haiti), equivalent to the Tireo Fm (Cordillera Central) in the Dominican Republic. The calc-alkaline, low-K tonalite shows LREE enrichment, and HREE depletion and have geochemical features similar to adakites, including 67.11 wt.% SiO, high NaO contents (3.69 wt.%), and high Sr/Y (38.6). U-Pb SHRIMP zircon dating yielded a concordant Pb/U emplacement age of 88.9 ± 1.1 Ma, similar to other calc-alkaline intrusions in Hispaniola and Cuba related to the Cretaceous GAA. We link the presence of rocks with adakitic affinities in Eastern Cuba (La Corea and Sierra del Convento Mélanges), Haiti (Massif du Nord; present study), and Dominican Republic (Cordillera Central) with subduction of the Proto-Caribbean ridge and eastward (present coordinates) migration of the corresponding ridge-trench-trench triple junction since at least ~120 Ma in eastern Cuba to the mid-Cretaceous (~90 Ma) in Hispaniola. Here we show that this migration is not consistent with left-lateral oblique subduction and propose that it was not characterized by steady motion but by sudden jumps resulting from the segmentation of the proto-Caribbean ridge by transform faults. Ridge subduction and jumping triple point migration should have been active during the Late Cretaceous to early Tertiary, when the activity of the Proto-Caribbean ridge vanished.

The dynamic tectonic history of peninsular Baja California commenced at approximately 13 Ma, when subduction along the northwestern coast of Mexico ceased. Subsequently, the Gulf of California opened, and transverse faults formed parallel to the ancestral trench. Post-subduction arc-related magmatism continued while the Baja peninsula was forming until approximately one million years ago. The post-subduction lavas that erupted in the peninsula have variable compositions ranging from calc-alkalic to tholeiitic arc basalts. Bajaite is a collective term that describes andesites and basaltic andesites found in Baja California that, like adakites, are characterized by elevated magnesium, silica, and incompatible trace element concentrations. However, in contrast, bajaites exhibit relative enrichment in Sr—their origin is the subject of discussion. It has been proposed as the product of partial melting of subducted oceanic crust - similar to the proposed origin of adakite - or partial melting of the metasomatized mantle wedge due to its arc lava-like features, such as depletion in Nb and Ta, and relative enrichment in Sr. Bajaite has also been proposed as a mixture of differentiated and mafic arc lavas. The composition of bajaite is similar to the bulk continental crust and, thus, its true origin can explain a potential mechanism of continental growth. In this study, I use the petrographic, major and trace element, and Sr-Nd-Pb isotopic data for bajaites from the Jaraguay and San Borja volcanic fields in Baja California Norte, Mexico to show that their source is plausibly the depleted mantle wedge metasomatized by a subduction component.

Whole-rock geochemical and Sr-Nd isotopic data are presented for late Miocene volcanic rocks associated with the Chah Zard epithermal Au-Ag deposit in the Urumieh-Dokhtar Magmatic Arc (UDMA), Iran, to investigate the magma source, petrogenesis and the geodynamic evolution of the study area. The Chah Zard andesitic to rhyolitic volcanic rocks are characterized by significant Large Ion Lithophile Element (LILE) and Light Rare Earth Element (LREE) enrichment coupled with High Field Strength Element (HFSE) depletion. Our geochemical data indicate an adakitic-like signature for the volcanic rocks (e.g. SiO2 > 62 wt%, Al2O3 > 15 wt%, MgO < 1.5 wt%, Sr/Y > 70, La/Yb > 35, Yb < 1 ppm, and Y < 18 ppm, and no significant Eu anomalies), distinguishing them from the other volcanic rocks of the UDMA. The Chah Zard volcanic rocks have similar Sr and Nd isotopic compositions; the Sr-87/Sr-86(i) ratios range from 0.704 902 to 0.705 093 and the epsilon(Nd)(i) values are from +2.33 to +2.70. However, the rhyolite porphyry represents the final stage of magmatism in the area and has a relatively high Sr-87/Sr-86 ratio (0.705 811). Our data suggest that the andesitic magmas are from a heterogeneous source and likely to result from partial melting of a metasomatized mantle wedge associated with a mixture of subducted oceanic crust and sediment. These melts subsequently underwent fractional crystallization along with minor amounts of crustal assimilation. Our study is consistent with the model that the volcanic host rocks to epithermal gold mineralization in the UDMA are genetically related to late Miocene Neo-Tethyan slab break-off beneath Central Iran.

The dynamic tectonic history of peninsular Baja California commenced at approximately 13 Ma, when subduction along the northwestern coast of Mexico ceased. Subsequently, the Gulf of California opened, and transverse faults formed parallel to the ancestral trench. Post-subduction arc-related magmatism continued while the Baja peninsula was forming until approximately one million years ago. The post-subduction lavas that erupted in the peninsula have variable compositions ranging from calc-alkalic to tholeiitic arc basalts. Bajaite is a collective term that describes andesites and basaltic andesites found in Baja California that, like adakites, are characterized by elevated magnesium, silica, and incompatible trace element concentrations. However, in contrast, bajaites exhibit relative enrichment in Sr—their origin is the subject of discussion. It has been proposed as the product of partial melting of subducted oceanic crust - similar to the proposed origin of adakite - or partial melting of the metasomatized mantle wedge due to its arc lava-like features, such as depletion in Nb and Ta, and relative enrichment in Sr. Bajaite has also been proposed as a mixture of differentiated and mafic arc lavas. The composition of bajaite is similar to the bulk continental crust and, thus, its true origin can explain a potential mechanism of continental growth. In this study, I use the petrographic, major and trace element, and Sr-Nd-Pb isotopic data for bajaites from the Jaraguay and San Borja volcanic fields in Baja California Norte, Mexico to show that their source is plausibly the depleted mantle wedge metasomatized by a subduction component.

Back-arc spreading centers initiate as fore-arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting. Two models have been proposed for triggering fore-arc/arc rifting: rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone. This paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high-viscosity boundary, which separates the weak fore arc from the hot buoyant mantle wedge, is removed. Buoyant upwelling mantle causes arc rifting, drives the overriding plate away from the subducting plate, and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism. For spontaneous fore-arc/arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces.

Back-arc spreading centers initiate as fore-arc or arc rifting events when extensional forces localize within lithosphere weakened by hydrous fluids or melting. Two models have been proposed for triggering fore-arc/arc rifting: rollback of the subducting plate causing trench retreat or motion of the overriding plate away from the subduction zone. This paper demonstrates that there is a third mechanism caused by an in situ instability that occurs when the thin high-viscosity boundary, which separates the weak fore arc from the hot buoyant mantle wedge, is removed. Buoyant upwelling mantle causes arc rifting, drives the overriding plate away from the subducting plate, and there is sufficient heating of the subducting plate crust and overriding plate lithosphere to form adakite or boninite volcanism. For spontaneous fore-arc/arc rifting to occur a broad region of weak material must be present and one of the plates must be free to respond to the upwelling forces.

Turrialba volcano lies in the southern sector of the Central American Volcanic Front (CAVF) in Costa Rica. The geochemistry of major and trace elements, and Sr and Nd isotopes of a selected suite of volcanic rocks ranging in composition from basaltic andesite to dacite and belonging to the last 10ka of activity of Turrialba volcano is described, together with the He-, Ne-, and Ar-isotope compositions of fluid inclusions hosted in olivine and pyroxene crystals. Most of the variability in the rock chemistry is consistent with typical trends of fractional crystallization, but there is an outlying group of andesites that displays an adakite-like composition (with a consistent depletion in high-field-strength elements and a marked enrichment in Sr) and low 3He/4He ratios (7.0-7.2 Ra). The trace-element composition of these rocks is typical of subduction-related magmas influenced by an OIB-like component at the source associated with the subduction of the Galapagos seamounts. The 87Sr/86Sr (0.703612-0.703678) and 143Nd/144Nd (0.512960-0.512984) ratios of the bulk rocks vary within narrow ranges, and are among the least-radiogenic isotope signatures of the CAVF volcanoes. The 3He/4He ratios measured in fluid inclusions hosted in olivine crystals (up to 8.1 Ra) are among the highest for the CAVF, and indicate that radiogenic 4He from fluids derived from the subducting slab contribute negligibly to the mantle wedge. The difference in He isotopes between most of studied rocks and those showing adakite-like features reasonably reflects two distinct components in the local mantle: (1) a MORB-like component, characterized by the highest He-isotope ratios (7.8-8.1 Ra), and (2) an OIB-like component, characterized by lower He-isotope ratios (7.0-7.2 Ra), coming from the subduction of the Galapagos seamounts. An overview at the regional scale indicates that high He-isotope ratios are peculiar to the two extreme sectors of the CAVF (Costa Rica to the south and Guatemala to the north

Coeval potassic adakite-like and shoshonitic felsic intrusions in the western Yunnan province of SW China are spatially and temporally associated with Eocene–Oligocene shoshonitic mafic volcanic rocks. The shoshonitic syenite and quartz monzonite intrusions are characterized by high K2O contents (4•9–6•8 wt %) and K2O/Na2O ratios (1•1–1•7), high Y (1•7–34•8 ppm) and Yb (1•50–3•16 ppm) contents, nearly flat heavy rare earth element (HREE) patterns and moderate Eu anomalies (Eu/Eu* = 0•65–0•78). The potassic adakite-like granite and quartz monzonite intrusions are characterized by enrichment in light rare earth elements (LREE), depletion in HREE and fractionated HREE patterns, high Sr (328–1423 ppm), Sr/Y (38–243) and La/Yb (23–62), and low Y and Yb contents. The shoshonitic syenite and quartz monzonites have the same Sr–Nd–Pb isotope compositions as the shoshonitic mafic volcanic rocks. They define linear trends on Harker diagrams, and have similar REE and trace element patterns to the shoshonitic mafic volcanic rocks. These observations suggest that the shoshonitic syenite and quartz monzonite magmas were differentiated from parental shoshonitic mafic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas originated from a metasomatized lithospheric mantle source.The shoshonitic syenite and quartz monzonites have higher magmatic zircon δ18O values (6•26–7•05‰) than the mantle, which suggests some 18O enrichment during earlier subduction-related metasomatism of their lithospheric mantle source. The potassic adakite-like granites have Sr–Nd–Pb isotopic compositions that overlap those of lower-crustal amphibolites. They have low Mg#, MgO, Ni and Cr contents, abundant inherited zircons, high zircon εHf (0–5•5) and mantle-like δ18O (4•78–6•25‰) values. These granites were plausibly derived by partial melting of a thickened, potassic, mafic, lower crust with minor input from an older igneous felsic component. The potassic adakite-lik

The Gangdese Belt in southern Tibet (GBST) is a major Cu–Au–Mo mineralization zone that mostly formed after the India–Asia collision in association with the small-volume, though widespread, Miocene(18–10 Ma) adakitic porphyries. Cu–Au mineralization has scarcely been found in the regional Jurassic–Early Tertiary batholiths related to subduction of the Neo-Tethyan oceanic plate. Here, we report petrological, zircon geochronological and geochemical data for Late Cretaceous (90 Ma) intrusive rocks that contain Cu–Au mineralization from the Kelu area in the GBST. These rocks consist of quartz monzonites and diorites. The quartz monzonites, with SiO2 of 58–59 wt.% and Na2O/K2O of 1.1–1.2, are geochemically similar to slab-derived adakites characterized by apparent depletions in heavy rare earth elements (e.g., Yb = 1.4–1.5 ppm) and Y (16–18 ppm) contents, positive Sr but negative Nb and Ti anomalies on multi element variation diagrams. They have relatively low (87Sr/86Sr)i (0.7038–0.7039) ratios and high εNd(t) (+3.4 to +3.9) and in situ zircon εHf(t) (+9.3 to +15.8) values.The diorites exhibit high Mg-numbers (0.57–0.61) similar to those of magnesian andesites, and have (87Sr/86Sr)i (0.7040–0.7041) and εNd(t) (+3.0 to +4.4) values similar to those of the quartz monzonites. We suggest that the quartz monzonitic magmas were most likely generated by partial melting of the subducted Neo-Tethyan basaltic oceanic crust and minor associated oceanic sediments, with subsequent melt–mantle interaction, and the dioritic magmas were mainly derived by the interaction between slab melts and mantle wedge peridotites, with fractionation of apatite and hornblende. These slab-derived adakitic magmas have high oxygen fugacity that may have facilitated Cu–Au mineralization. The close association of the Late Cretaceous adakitic intrusive rocks and Cu–Au mineralization in the Kelu area suggests that the arc magmatic rocks in the GBST may have higher potential than previously thought for Cu–A

The Palaeoproterozoic Svecofennian orogen in the Fennoscandian shield is an arc accretionary orogen that was formed at c. 1.92-1.86Ga. Arc accretion, magmatism and the subsequent continent-continent collision thickened the crust up to c. 70km, forming one of the thickest Palaeoproterozic orogens. At the end stage of accretionary tectonics, voluminous synorogenic magmatism occurred in southwestern Finland leading to the intrusion of intermediate to felsic plutonic rocks. Ion microprobe single zircon dating of one diorite sample yielded an age of 1872 +/- 3Ma (epsilon(Nd)=+2.2) and the trondhjemite sample an age of 1867 +/- 4Ma (epsilon(Nd)=+2.6). Inherited 2667-1965Ma cores and 1842 +/- 5Ma metamorphic rims were also found in zircons from the trondhjemite. The dioritic magmatism is mantle-derived and is slightly enriched by subduction-related processes. The felsic magmatism shows elevated Sr/Y and La/Yb ratios, which are typical for adakite- and TTG-like magmas. Their low Mg#, Ni and Cr contents argue against slab-melting and mantle-wedge contamination. We infer that the felsic magmatism was generated through crustal melting of the lower part of the previously generated volcanic-arc type crust. Based on published melting experiments and the Sr and Y contents of the felsic rocks we suggest that the melts were generated at a minimum pressure of 10kbar, with evidence of a 15kbar pressure for the highest Sr/Y trondhjemites. It is proposed that arc accretion combined with magmatic intrusions thickened the crust so that melting of the lower crust yielded adakite- and TTG-like compositions. The mafic magmatism is considered to be the heat source.

Subduction of the Farallon plate beneath northwestern Mexico stalled by ~12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (~24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ~40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ~8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes.

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 39 (2012): L09307, doi:10.1029/2011GL050828., Subduction of the Farallon plate beneath northwestern Mexico stalled by ~12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (~24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ~40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ~8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes., This work was funded by the NSF Margins Program, grant number OCE-0112058., 2012-11-08

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 39 (2012): L09307, doi:10.1029/2011GL050828., Subduction of the Farallon plate beneath northwestern Mexico stalled by ~12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (~24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ~40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ~8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes., This work was funded by the NSF Margins Program, grant number OCE-0112058., 2012-11-08

Author Posting. © American Geophysical Union, 2012. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 39 (2012): L09307, doi:10.1029/2011GL050828., Subduction of the Farallon plate beneath northwestern Mexico stalled by ~12 Ma when the Pacific-Farallon spreading-ridge approached the subduction zone. Coupling between remnant slab and the overriding North American plate played an important role in the capture of the Baja California (BC) microplate by the Pacific Plate. Active-source seismic reflection and wide-angle seismic refraction profiles across southwestern BC (~24.5°N) are used to image the extent of remnant slab and study its impact on the overriding plate. We infer that the hot, buoyant slab detached ~40 km landward of the fossil trench. Isostatic rebound following slab detachment uplifted the margin and exposed the Magdalena Shelf to wave-base erosion. Subsequent cooling, subsidence and transtensional opening along the shelf (starting ~8 Ma) starved the fossil trench of terrigenous sediment input. Slab detachment and the resultant rebound of the margin provide a mechanism for rapid uplift and exhumation of forearc subduction complexes., This work was funded by the NSF Margins Program, grant number OCE-0112058., 2012-11-08

The Palaeoproterozoic Svecofennian orogen in the Fennoscandian shield is an arc accretionary orogen that was formed at c. 1.92-1.86Ga. Arc accretion, magmatism and the subsequent continent-continent collision thickened the crust up to c. 70km, forming one of the thickest Palaeoproterozic orogens. At the end stage of accretionary tectonics, voluminous synorogenic magmatism occurred in southwestern Finland leading to the intrusion of intermediate to felsic plutonic rocks. Ion microprobe single zircon dating of one diorite sample yielded an age of 1872±3Ma (eNd=+2.2) and the trondhjemite sample an age of 1867±4Ma (eNd=+2.6). Inherited 2667-1965Ma cores and 1842±5Ma metamorphic rims were also found in zircons from the trondhjemite. The dioritic magmatism is mantle-derived and is slightly enriched by subduction-related processes. The felsic magmatism shows elevated Sr/Y and La/Yb ratios, which are typical for adakite- and TTG-like magmas. Their low Mg#, Ni and Cr contents argue against slab-melting and mantle-wedge contamination. We infer that the felsic magmatism was generated through crustal melting of the lower part of the previously generated volcanic-arc type crust. Based on published melting experiments and the Sr and Y contents of the felsic rocks we suggest that the melts were generated at a minimum pressure of 10kbar, with evidence of a 15kbar pressure for the highest Sr/Y trondhjemites. It is proposed that arc accretion combined with magmatic intrusions thickened the crust so that melting of the lower crust yielded adakite- and TTG-like compositions. The mafic magmatism is considered to be the heat source.

The Palaeoproterozoic Svecofennian orogen in the Fennoscandian shield is an arc accretionary orogen that was formed at c. 1.92-1.86Ga. Arc accretion, magmatism and the subsequent continent-continent collision thickened the crust up to c. 70km, forming one of the thickest Palaeoproterozic orogens. At the end stage of accretionary tectonics, voluminous synorogenic magmatism occurred in southwestern Finland leading to the intrusion of intermediate to felsic plutonic rocks. Ion microprobe single zircon dating of one diorite sample yielded an age of 1872±3Ma (eNd=+2.2) and the trondhjemite sample an age of 1867±4Ma (eNd=+2.6). Inherited 2667-1965Ma cores and 1842±5Ma metamorphic rims were also found in zircons from the trondhjemite. The dioritic magmatism is mantle-derived and is slightly enriched by subduction-related processes. The felsic magmatism shows elevated Sr/Y and La/Yb ratios, which are typical for adakite- and TTG-like magmas. Their low Mg#, Ni and Cr contents argue against slab-melting and mantle-wedge contamination. We infer that the felsic magmatism was generated through crustal melting of the lower part of the previously generated volcanic-arc type crust. Based on published melting experiments and the Sr and Y contents of the felsic rocks we suggest that the melts were generated at a minimum pressure of 10kbar, with evidence of a 15kbar pressure for the highest Sr/Y trondhjemites. It is proposed that arc accretion combined with magmatic intrusions thickened the crust so that melting of the lower crust yielded adakite- and TTG-like compositions. The mafic magmatism is considered to be the heat source.

This study reports age, petrologic and geochemical data for andesites and dacites from the Late Triassic sedimentary strata of northern Hohxil, in the Hohxil–Songpan–Ganzi Block (northern Tibet), which constitutes the most voluminous Triassic strata on Earth. LA-ICP-MS zircon U-Pb analysis of dacite (210.4±1.9 Ma) and whole rock 40Ar–39Ar analyses for both the andesites and dacites (211±2 Ma and 210.9±1.6 Ma) show that the rocks were almost contemporaneous. Both rock types are sodium-rich and calc-alkaline. The andesites, characterized by high MgO (up to 10 wt.%) or Mg* (~70), TiO2, Al2O3, Cr, Ni, La/Yb and Th/La, but low Nb/Ta ratios, are geochemically similar to sanukitoids in southeastern Japan. The dacites are strongly peraluminous, and have high Al2O3 and La/Yb, low Y and Yb, coupled with negligible to positive Eu and Sr anomalies, comparable to slab-derived adakites in the circum Pacific arc system. Both rocks exhibit strongly fractionated platinum group element patterns, with Pt/Pt* (Pt anomaly), (Pt/Ir)N and Re/Os ratios higher than those of the primitive mantle. They have uniformly low εNd(t) values (-7.57–-9.59) and high (86Sr/87Sr)i ratios (0.7086–0.7106) that imply a continental rather than oceanic type magma source. We suggest that the northern Hohxil dacites were produced by partial melting of subducted sediments on the northward-subducting Songpan–Ganzi oceanic slab, and the high-Mg andesites were formed by subsequent interaction between the sediment-derived melts and mantle wedge peridotites. Taking into account the Triassic magmatic record from nearby regions, we suggest that the Late Triassic high-Mg andesite/dacite suites of northern Hohxil were generated in a forearc setting, and propose that double-sided subduction eventually closed the Songpan–Ganzi ocean during the Late Triassic.

Voluminous Late Paleozoic igneous rocks and associated Cu–Au–Mo deposits occur in the northwestern Tianshan district, Xinjiang, west China. However, the tectonic setting and petrogenesis of these rocks remain controversial. This paper reports zircon U–Pb and Hf CUU isotopic data, major and trace elements, and Sr–Nd–Pb isotopic data for the intrusive rocks and minor dacites in the Lamasu–Dabate area of northwestern Tianshan adjacent to the Cu–Au–Mo deposits. LA-ICPMS U–Pb zircon analyses suggest that the Lamasu porphyries were formed at 366 ± 3 Ma and contain 907–738 Ma inherited zircons, the Dabate dacites were formed at 316 ± 4 Ma, and granite porphyries were formed at 289 ± 3 Ma with ~ 319 Ma inherited zircons. The Lamasu porphyries consist of plagioclase granite and granodiorite, and are geochemically similar to adakites, e.g., having high Al2O3 (14.54–19.75 wt.%) and Sr (308–641 ppm) and low Y (7.84–16.9 ppm) contents, with fractionated rare earth element (REE) patterns and slightly positive Sr anomalies.However, they have variable initial ratios of 87Sr/86Sr (0.7072–0.7076) and 206Pb/204Pb (18.139–18.450), and variable εNd(t) (− 5.6 to − 0.8) and positive εHf(t) (+ 1.4 to + 10.6) values. They also have variable Mg# (100 × Mg2+/(Mg2++ Fe2+)) (41–73) and low Th (3.13–8.09) and Th/Ce (0.14–0.28) values. We suggest that the Lamasu adakitic magmas were generated through partial melting of southward subducted Junggar oceanic crust, with subsequent melt-mantle interaction and assimilation of basement rocks. The Dabate dacites show typical arc-like geochemical characteristics (e.g., enrichment of large ion lithophile elements (LILE) and strong negative anomalies of Ta, Nb, P and Ti), with variable εNd(t) (+ 0.1 to + 3.3). They were probably generated by melting of juvenile basaltic lower crust as a result of magma underplating.The Dabate granite porphyries are geochemically similar to A2-type granites, e.g., high SiO2 (75.6–77.6 wt.%) and alkalis (Na2O + K2O = 8.27–8.7

The Central Asian Orogenic Belt (CAOB) is a natural laboratory for the study of accretionary tectonics and crustal growth owing to its massive generation of juvenile crust in the Paleozoic. There is a debate, however, on the mechanism of this growth. In the Baogutu area of the western Junggar region, northern Xinjiang (west China), diorite–granodiorite porphyry plutons and dikes are widely associated with Cu–Au mineralization. In this study, we present new results of zircon U–Pb geochronology, major and trace elements, and Sr–Nd–Pb–Hf isotope analyses for two diorite–granodiorite porphyry plutons and two dikes from this area. LA-ICP-MS zircon U–Pb analyses of four plutonic and dike samples yield Late Carboniferous ages of 315–310 Ma. The Baogutu diorite–granodiorite porphyries exhibit low-Fe and calc-alkaline compositions.They are also characterized by high Sr (346–841 ppm) contents, low Y (9.18–16.5 ppm) and Yb (0.95–1.60 ppm) contents, and relatively high Sr/Y (31–67) ratios, which are similar to those of typical adakites. In addition, some samples have relatively high MgO (2.35–8.32 wt.%) and Mg# (48–75), and Cr (22.7–291 ppm) and Ni (32.0–132 ppm) values, which are similar to those of high-Mg andesites. All rock samples exhibit mid-oceanic ridge basalt (MORB)-like Nd–Sr–Pb–Hf isotope features: high εNd(t) (+ 5.8–+8.3) and εHf(t) (+ 13.1–+15.7) values, and relatively low (87Sr/86Sr)i (0.7033 to 0.7054) and (206Pb/204Pb)i (17.842–18.055). The Baogutu adakitic rocks also contain reversely zoned clinopyroxene phenocrysts, which have low MgO cores and relatively high MgO rims. Geochemical modeling indicates that the Baogutu adakitic rocks could have been derived by mixing ~ 95% altered oceanic crust-derived melts with ~ 5% sediment-derived melts.Taking into account the regional geology, I- and A-type granitoids and Cu–Au mineralization, and the presence of Carboniferous ophiolite mélanges in northern Xinjiang, we suggest that the Baogutu adakitic rocks were most prob

Las intrusiones granodioríticas que dieron origen a un depósito de Au-Cu tipo skarn en el distrito minero de La Paz, S.L.P., se localizan en la parte oriental de la Mesa Central. El skarn se desarrolló en el contacto entre una secuencia sedimentaria calco-argílica del Cretácico medio y los intrusivos. Un sistema de vetas mineralizadas en Ag-Pb-Zn post-datan el Skarn. El distrito de La Paz está dividido por dos sistemas de fallas muy bien defi nidas (N-S y E-W). La falla Dolores, de dirección N-S , muestra Revista Mexicana de Ciencias Geológicas, v. 25, núm. 1, 2008, p. 39-58 un desplazamiento normal vertical estimado entre 500 a 1000 m y separa la zona occidental de skarn de Au-Cu de la zona oriental que contiene al sistema hidrotermal de vetas de Ag-Pb-Zn. Esta falla se considera como parte del sistema de fallas Taxco-San Miguel de Allende. El fechamiento de los intrusivos mediante el método U-Pb en circones indica claramente un único evento de emplazamiento alrededor de 37 Ma. Esta fecha representa el último de los pulsos mineralizantes, posteriores a la orogenia Laramide, reconocido en el distrito de la Sierra de Catorce. Asimismo se reportan cuatro edades discordantes que sugieren la presencia de rocas greenvilianas en la base de una corteza gruesa (35-45 km). La geoquímica de los intrusivos muestra algunas diferencias en su composición, pero pertenecen a la serie magmática calco-alcalina con alto contenido de K. Los estudios de elementos mayores y traza muestran una evolución desde el campo adakítico hasta el campo de arco de islas, desde los intrusivos mineralizados a los estériles, respectivamente. Estos datos también sugieren la importancia del proceso de delaminación cortical y la necesidad de fallas profundas para transferir dicho magma y metales hacia la superfi cie.

Las intrusiones granodioríticas que dieron origen a un depósito de Au-Cu tipo skarn en el distrito minero de La Paz, S.L.P., se localizan en la parte oriental de la Mesa Central. El skarn se desarrolló en el contacto entre una secuencia sedimentaria calco-argílica del Cretácico medio y los intrusivos. Un sistema de vetas mineralizadas en Ag-Pb-Zn post-datan el Skarn. El distrito de La Paz está dividido por dos sistemas de fallas muy bien defi nidas (N-S y E-W). La falla Dolores, de dirección N-S , muestra Revista Mexicana de Ciencias Geológicas, v. 25, núm. 1, 2008, p. 39-58 un desplazamiento normal vertical estimado entre 500 a 1000 m y separa la zona occidental de skarn de Au-Cu de la zona oriental que contiene al sistema hidrotermal de vetas de Ag-Pb-Zn. Esta falla se considera como parte del sistema de fallas Taxco-San Miguel de Allende. El fechamiento de los intrusivos mediante el método U-Pb en circones indica claramente un único evento de emplazamiento alrededor de 37 Ma. Esta fecha representa el último de los pulsos mineralizantes, posteriores a la orogenia Laramide, reconocido en el distrito de la Sierra de Catorce. Asimismo se reportan cuatro edades discordantes que sugieren la presencia de rocas greenvilianas en la base de una corteza gruesa (35-45 km). La geoquímica de los intrusivos muestra algunas diferencias en su composición, pero pertenecen a la serie magmática calco-alcalina con alto contenido de K. Los estudios de elementos mayores y traza muestran una evolución desde el campo adakítico hasta el campo de arco de islas, desde los intrusivos mineralizados a los estériles, respectivamente. Estos datos también sugieren la importancia del proceso de delaminación cortical y la necesidad de fallas profundas para transferir dicho magma y metales hacia la superfi cie.

The Early Miocene volcanic rocks (the Iwaine Formation) are widely distributed in the Hokuriku Green Tuff Province. The volcanic rocks in the Iwaine Formation in the southern part of Toyama Prefecture can be divided into two groups in the FeO^*/MgO-SiO_2 diagram；andesite group I has a tholeiitic trend showing enrichment in FeO^*/MgO ratio from mafic to felsic andesite, whereas andesite group II has a calc－alkaline trend on which SiO_2 content increases from about 54 to 68 wt.% despite a little variation in FeO^＊/MgO. Petrographic and chemical data indicate that andesite group I was formed from tholeiitic basalt magma by fractional crystallization of its phenocryst minerals. Andesite group II is composed mainly of calc－alkaline andesite with subordinate amounts of high－magnesian andesite and adakitic andesite. Most phenocrystic plagioclases in calc-alkaline andesite are dusy and show reverse zoning in terms of Ca, suggesting that the andesite was of magma mixing origin. Adakitic andesite is characterized by higher Sr and lower Y contents, and higher Sr/Y ratio than common andesites in island arcs, and is also characterized by higher MgO, Ni and Cr contents compared with typical adakite. The high-magnesian andesite shows lower K_2O and Rb contents, and higher Sr/Y ratio than the Setouchi high-magnesian andesite and boninite. Adakitic andesite may result from modification of felsic (adakitic) magma generated by partial melting of the subducted oceanic crust, due to interaction with the overlying mantle peridotite or mixing with mafic magma during its rise to the surface. The high-magnesian andesite seems to have a genetical close relation to adakitic andesite. K_2O content of basaltic magmas in the Iwaine Formation tends to increase from the study area (eastern margin of the Hokuriku Green Tuff Province) to the western area. Thus, it is possible to consider that the study area occupied the volcanic front in the Early Miocene Hokuriku Green Tuff Province., 富山県南部に分布する中新統岩稲累層の安山岩類は，8タイプの安山岩～デイサイトに分けられ，それらはFeO^＊/MgO-SiO_2図中で，ソレアイト系列の領域を占めるもの（グループ-1）と，カルクアルカリ系列～高マグネシア安山岩の領域を占めるもの（グループ-II）とに区分される．グループ-IIの安山岩はカルクアルカリ系列の安山岩，アダカイト質の安山岩，高マグネシア安山岩からなる．これらの安山岩類の成因は以下のように考えられる．グループ-Iの安山岩；低アルカリソレアイト質の玄武岩質マグマの分別結晶作用，グループ-IIのカルクアルカリ系列の安山岩；安山岩質マグマと玄武岩質マグマとの混合，アダカイト質の安山岩；沈み込んだ海洋地殻に由来するアダカイト質マグマと，マントルカンラン岩との反応あるいは玄武岩質マグマとの混合，高マグネシア安山岩；成因的にアダカイト質の安山岩と関連する，また，本地域は岩稲累層の火山活動域の火山フロントに相当する可能性があることを議論した．

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.

We report here the compositions of primitive high-Mg adakite lavas from the Kadavu Island Group, Fiji. Two distinctive high-Mg adakite compositions are present on Kadavu, and both are strongly enriched in Sr and La, with high Sr/Y (>120) and La/Yb (>30) values. The east Kadavu high-Mg adakite is relatively enriched in K2O and depleted in TiO2 and Nb. Our results from Kadavu suggest that there is an adakite magma series that is the result of complex open-system magmatic processes, including magma mixing between relatively low SiO2 mantle-derived melts of high-Mg adakite composition and high SiO2, low-Mg adakite magma. The results from Kadavu Island suggest that adakite magma suites worldwide are likely to include primitive high-Mg compositions, despite the lack of fi eld evidence in many cases.