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The Miocene Las Burras--Almagro--El Toro magmatic complex lies ~300 km to the east of the Central Andes volcanic arc, in the easternmost sector of the transverse Calama--Olacapato--El Toro fault zone. The magmatic rocks of the Las Burras--Almagro--El Toro complex comprise a monzogabbro to monzogranite laccolith like intrusion and basaltic andesite to dacite volcanic rocks that include seven lithostratigraphic members. New Rb-Sr dates indicate that the intrusive rocks are ca. 14 Ma, and K-Ar dates suggest emplacement ages of ca. 12.8-6.4 Ma for the volcanic rocks. The emplacement of the intrusion was controlled by N-S--striking strike-slip faults in a context of oblique convergence; the volcanism, which occurred along WNW-ESE--and N-S--striking extensional faults, relates to the Calama--Olacapato--El Toro fault zone. Two magmatic phases were recognized. Intrusive and volcanic rocks of the older magmatic phase (ca. 14-13 Ma) are characterized by Ba/Nb (7-14), La/Ta (11-18), and isotopic ratios ([sup.87]Sr/[sup.86]Sr: 0.704339-0.705281, [sup.143]Nd/[sup.144]Nd: 0.512713-0.512598), which are intraplate characteristics. The source of the older magmas was isotopically depleted lithospheric mantle rich in K, Rb, and Th. Energy constrained-assimilation and fractional crystallization (EC-AFC) modeling indicates that fractional crystallization and crustal assimilation moderately modified magma composition during its residence in the crust. The products of the younger magmatic phase (ca. 11-6 Ma) have higher Ba/Nb (24-42) and La/Ta (24-30) and [sup.87]Sr/[sup.86]Sr (0.706738-0.708729) and lower [sup.143]Nd/[sup.144]Nd (0.512433-0.512360). The results of EC-AFC modeling exclude a significant role for the upper crust in the generation of the most primitive magmas of this phase. Their compositions can be explained by (1) contamination of the primary magmas having originated in a depleted mantle with a mafic crust, or (2) the contribution of isotopically enriched mantle zones. Shallow differentiation and moderate contamination by continental crust can explain the composition of the intermediate and evolved products of the younger phase. The variation of the magma source characteristics at 11 Ma is discussed in the frame of the complex geodynamical setting in this region. Keywords: Central Andes, Eastern Cordillera, backarc magmatism, volcanic stratigraphy, geochronology, structural geology.

Cratons are old, stable parts of the continental crust that have survived at least since Proterozoic rime and have not undergone strong magmatism or tectonism since their stabilization. Traditionally, the growth of continental crust in the North China craton is considered to have been complete by Paleoproterozoic time, Phanerozoic events being largely restricted to surrounding orogenic belts. However, the eastern part of the North China craton contains large volumes of Mesozoic igneous rocks, with widespread metamorphic core complexes and pull-apart basins. Hf isotope compositions of magmatic zircon grains from igneous rocks in the Liaodong Peninsula indicate that widespread late Mesozoic granitoids formed by partial melting of ancient crust, but with significant input of a mantle component via magma mixing and crustal assimilation. This magmatism has resulted from removal and modification of lithospheric mantle, accompanied by asthenospheric upwelling. The Hf isotopic signatures thus record the addition of juvenile crust beneath the eastern part of the North China craton, which appears related to major extension and possibly slab rollback of the Pacific plate. Whatever the mechanism, it is apparent that since ca. 200 Ma, ancient lithosphere beneath the eastern North China craton has been progressively reactivated and replaced, resulting in 'decratonization.' Tertiary to Holocene volcanism in the area and major subsidence around the Gulf of Bohai suggest that the effects of the process are continuing. Similar processes probably operated in the geological past, leading to significant modification of continental crust and requiring reconsideration of mechanisms for continental breakup and dispersal and of continental growth rates. Keywords: decratonization, Phanerozoic crustal growth, North China craton, zircon, Hf isotope, magmatism.

Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identified on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%-50% of total extension is accommodated by magmatic accretion and that there is significant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges. Keywords: mid-ocean ridge, detachment fault, megamullion, oceanic core complex, oceanic magmatism, numerical modeling.

Volcanic and intrusive rocks of the Dhiarizos Group, occurring within the Mamonia Complex of SW Cyprus, comprise mainly tholeiitic basalts, ocean island tholeiites (OIT) and other alkalic varieties, with minor alkaline lamprophyre intrusive rocks. These rocks were highly tectonized and dismembered during the collision with the Troodos oceanic lithosphere. Trace element data for the most primitive mafic igneous rock samples suggest that the tholeiitic basalts are derived from a depleted mantle source whereas the OIT and the alkalic basalts originated from variable degrees of partial melting of a spinel peridotite mantle. Alkaline lamprophyres are products of a small degree of partial melting of a deep-seated garnet lherzolite. The bulk of the tholeiites and alkalic varieties are Late Triassic in age and are interpreted as having formed during Neo-Tethyan sea-floor spreading and associated seamount volcanism. Amphibole separates from a lamprophyre intrusion have [sup.40][Ar.sup.-39] Ar ages of 140.7 [+ or -] 0.4 Ma. In this paper, we report for the first time an Early Cretaceous alkaline lamprophyric magmatism, which is probably associated with a renewed episode of Gondwanaland rifting. These results show that the development of the Neo-Tethyan oceanic basin in the Eastern Mediterranean was largely controlled by a two-phase rifting--drifting process.

Recharge by mafic magma is established as a trigger of major explosive volcanic eruptions. However, recharge by silicic magma can also induce perched resident magma to erupt. The evidence of such recharge may be more cryptic than for mafic recharge and requires detailed petrologic and geochemical study to identify. We present a study of plagioclase petrology supported by other data that implicates silicic-silicic magma interaction during the eruption and evolution of the A.D. 1600 eruption of Huaynaputina, one of the largest historical eruptions. Recharge by silicic magma is probably more common than recognized and should be considered an important eruption trigger. Keywords: silicic recharge, crystal mush, magma mingling, eruption trigger, Huaynaputina.

Extension during plate convergence and mountain building is widely recognized, yet the causes of synconvergent extension remain controversial. Here we propose that delamination of lithospheric mantle, aided by decoupling of the crust from the mantle via a reduction in the viscosity of the lower crust through heating, incursion of fluids, and partial melting, explains many enigmatic yet prevalent aspects of the metamorphic, magmatic, and kinematic history of the Sevier-Laramide orogen of the western United States during the Late Cretaceous. Extension, heating, anatexis, magmatism, and perhaps rock uplift were widespread during a restricted time interval in the Late Cretaceous (75-67 Ma) along the axis of maximum crustal thickening within the Mojave sector of the Sevier orogen, and to a lesser extent within the interior of the Idaho-Utah-Wyoming sector to the north; similar processes may have been active in the Peninsular Range, Sierran, western Mojave, and Salinian segments of the Mesozoic Cordilleran arc. These processes are viewed as predictable consequences of the thermal, rheological, and dynamic state of the overlying crust following delamination of mantle lithosphere beneath isostatically compensated mountain belts. The proposed delamination would have occurred immediately prior to eastward propagation of low angle subduction of the Farallon plate during the inception of the Laramide orogeny. Following delamination, extension and anatexis of the North American crust were aided locally by egress of slab-derived fluids from the low-angle Farallon slab. We suggest that lithospheric delamination may have aided in the shallowing of the slab to achieve lowangle subduction geometry. Delamination has been proposed to be common in areas of thickened continental lithosphere in the terminal phase or late in orogenesis. The Late Cretaceous delamination event proposed here for the Sevier-Laramide orogen occurred during protracted plate convergence and was synchronous with, and followed by, continued shortening in the external part of the orogen. Keywords: Cordilleran tectonics, Late Cretaceous, delamination, synconvergent extension, Laramide.

Miocene volcanic rocks in the Pinacate area, Sonora, record a progressive change in the source of magmatism induced by asthenospheric upwelling and lithospheric thinning. [sup.40] Ar/[sup.39] Ar age data, mineral chemistry, and major- and trace-element contents allow the identification of two volcanic sequences: an oldest basaltic episode (ca. 20 Ma), and a middle Miocene (12-15.5 Ma) sequence that consists of mesa basalts with transitional alkali character, calc-alkaline dacites, and high-silica rhyolites evolving toward peralkaline liquids. Sr, Nd, and Pb isotope ratios reveal different sources for the Miocene basalts. The easternmost basalts have signatures indicating a Precambrian lithospheric mantle source, while the westernmost tholeiitic to transitional basalts are related to mixing of lithospheric and asthenospheric mantle. Rhyolites are the result of fractional crystallization of transitional basalt magmas with slight contamination by Precambrian crust. Chemical modeling shows that peral kaline rhyolites are related to slightly higher assimilation during their residence in the upper crust but also to a change in the mantle source of the parent basalt. The evolution of the isotopic signatures in space and time indicates that: (1) the volcanic activity is located over a major lithospheric boundary, i.e., the western limit of the North American Craton; (2) the lithosphere was progressively thinned so that huge volumes of alkalic basalts could access the surface during the Quaternary, building the Pinacate Volcanic Field. Correlation between geochemical signatures and the tectonic evolution of the western margin of the North American Craton shows that a progressive change in the source of magmatism can be related to the development of a slab window during the Miocene. Keywords: Volcanism, Mexico, geochronology, petrology, geochemistry, isotopes.

Forty-three [sup.40] Ar/[sup.39] Ar age determinations of lava flows, domes, ignimbrites, and dikes, plus [sup.14]C dates from seven distal tephra layers, combined with stratigraphy, geochemistry, and Sr and Th isotope data, establish an eruptive chronology for the PuyehueCordon Caulle volcanic complex at 40.5[degrees] S in the Andean southern volcanic zone (SVZ). The complex preserves ~131 [km.sup.3] of lava and tephra that erupted from numerous vents widely separated in time and space. Approximately 80% of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka. The modern Puyehue stratovolcano was built on the southerly shield during the past 69 k.y. following a hiatus of 25 k.y. Puyehue has erupted ~15 [km.sup.3] of basaltic to rhyolitic magma that spans the entire compositional range found in the southern SVZ and evolved via at least six phases including: (1) basaltic andesitic to dacitic lavas between 69 and 32 ka, (2) a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of MgO-rich basaltic andesite at 34 ka, (3) dacitic to rhyolitic flows and domes from 19 to 12 ka, (4) basaltic to basaltic andesitic flows between 15 and 12 ka, (5) subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka, followed by (6) a powerful series of phreatomagmatic and sub-Plinian eruptions at ca. 1.1 ka that obliterated the preceding rhyolite domes and formed the present 2.5km-diameter, 280-m-deep summit crater. Along the Cordon Caulle fissure zone, ~5 [km.sup.3] of rhyodacitic to rhyolitic lavas, domes, and cones have formed during the past ~16.5 k.y., including explosive and effusive eruptions in 1921-1922 and 1960. Eruptive rates were nonuniform over time, with background growth at 0.04 [km.sup.3]/k.y, or less, punctuated by spurts at up to 0.90 [km.sup.3]/ k.y. The time-averaged rate, 0.42 [km.sup.3]/k.y., is nearly double that at the Tatara-San Pedro complex 500 km to the north during the past 300 k.y. These findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable. The last three stratoeone-building events on Puyehue began during periods of deglaciation, suggesting a relationship between unloading of ice and ease of magma ascent. Puyehue basalt exhibits subtle changes in [sup.238]U-[sup.230]Th, [sup.87]Sr/[sup.86] Sr, and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge, or the extent to which basalt was modified by assimilation of heterogeneous crustal melts. The complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt. Despite the high flux of basalt during the past 300 k.y., no large silicic magma reservoir formed in the upper crust. Instead, [sup.238]U-[sup.230] Th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust, promoted perhaps by conduits that reflect strike-slip faulting beneath the complex. Keywords: Andes, volcanism, Puyehue, geochronology, [sup.40] Ar/ [sup.39] Ar dating.

The Rabinal-Salama area in central Guatemala provides critical data bearing on the relationships between the North American and Caribbean plates because it lies within the Polochic-Motagua fault zone that separates the two plates. The cumulative Cenozoic sinistral displacement across this zone that separates the Maya and Chortis terranes has been variously estimated to be ~125 km or -1100 km, evidence for which should be recorded in the rocks of the studied area. The Rabinal-Salama area lies between two of the east-west faults within the Motagua fault zone, the Polochic fault, and the Baja Verapaz shear zone. The shear zone separates the Maya block from eclogitic rocks of the Chuacus Complex that pass southward into ophiolitic rocks and me1anges that define a suture between the Chuacus Complex and the Chortfs block. The following sequence of events is recorded in the Rabinal-Salama area: (1) lowgrade, pre-Silurian siliciclastic metasedimentary rocks (San Gabriel unit), that are intruded by (2) ca. 462-453 Ma caic-alkaline, peraluminous, S-type Rabinal granite suite, and unconformably overlain by (3)very low grade clastic and calcareous metasedimentary rocks (Santa Rosa Group) contain ing Mississippian conodonts and pebbles of granite, sandstone, and phyllite derived from the older units. The Rabinal granite suite is inferred to be rift related, inheriting its calc-alkaline signature from its source, along with the ca. 1 Ga xenocrystic zircons (upper intercept U-Pb data). Deformation in all these Paleozoic rocks produced a steeply south-southwest-dipping cleavage (chlorite and sericite) and a stretched quartz lineation. These fabrics become more intense adjacent to the Baja Verapaz shear zone, where C-S fabrics and rotated porphyroclasts indicate a reverse sense of motion with a sinistral component. White mica in the shear zone yields 74-65 Ma K-Ar ages, which are inferred to closely postdate the time of crystallization. Thus, although evidence for major sinistral displacement is absent, the kinematics are consistent with uplift and exhumation of the Chuacus Complex during obduction of the Baja Verapaz ophiolite onto the Paleozoic rocks of the Rabinal-Salama area in latest Mesozoic-Paleocene. This is inferred to have been produced during collision of the Cuban arc and Chortis block with the southern Maya block. Restoration of the Early Mesozoic ~70[degrees] anticlockwise rotation of the Maya block places the Rabinal-Salama area adjacent to northeastern Mexico, where comparable continental-shallow marine, Paleozoic rocks occur near Ciudad Victoria overlying the ca. 1 Ga Oaxaquia basement. Keywords: Guatemala, Maya block, Chortis block, Caribbean tectonics, Motagua, Paleozoic, Rabinal granite.

The Skymo Complex in the North Cascades, Washington, is a layered mafic intrusion within the Ross Lake fault zone, a major orogen-parallel structure at the eastern margin of the Cascades crystalline core. The complex is composed dominantly of troctolite and gabbro, both with inclusions of primitive olivine gabbro. Low-pressure minerals in the metasedimentary contact aureole and early crystallization of olivine + plagioclase in the mafic rocks indicate the intrusion was emplaced at shallow depths ( The Skymo rocks have trace-element characteristics of arc magmas, but the association of Mg-rich olivine ([Fo.sub.88-80]) with relatively sodic plagioclase ([An.sub.75-60]) and the Al/Ti ratios of clinopyroxene are atypical of arc gabbros and more characteristic of rift-related gabbros. A Sm-Nd isochron indicates crystallization in the early Tertiary (ca. 50 Ma), coeval with the nearby Golden Horn alkaline granite. Mantle melting to produce Skymo magma likely occurred in a mantle wedge with a long history of arc magmatism. The Skymo mafic complex and the Golden Horn granite were emplaced during regional extension and collapse of the North Cascades orogen and represent the end of large-scale magmatism in the North Cascades continental arc. Keywords: gabbro, layered marie intrusion, North Cascades, orogenic collapse, Skymo Complex.

[1] Tectonic activity in convergent plate boundaries commonly involves backward migration (rollback) of narrow subducting slabs and segmentation of subduction zones through slab tearing. Here we investigate this process in the Italian region by integrating seismic tomography data with spatiotemporal analysis of magmatic rocks and kinematic reconstructions. Seismic tomography results show gaps within the subducting lithosphere, which are interpreted as deep (100-500 km) subvertical tear faults. The development of such tear faults is consistent with proposed kinematic reconstructions, in which different rates of subduction rollback affected different parts of the subduction zone. We further suggest a possible link between the development of tear faults and the occurrence of regional magmatic activity with transitional geochemical signatures between arc type and OIB type, associated with slab tearing and slab breakoff. We conclude that lithosphericscale tear faults play a fundamental role in the destruction of subduction zones. As such, they should be incorporated into reconstructions of ancient convergent margins, where tear faults are possibly represented by continental lineaments linked with magmatism and mineralization. Citation: Rosenbaum, G., M. Gasparon, F. P. Lucente, A. Peccerillo, and M. S. Miller (2008), Kinematics of slab tear faults during subduction segmentation and implications for Italian magmatism, Tectonics, 27, TC2008, doi:10.1029/2007TC002143.

High-flux pulses of magmatism that make up most of the exposed North American Cordilleran arcs are derived primarily from upper plate lithospheric source materials, and not the mantle wedge as most models would predict, based on a compilation of thousands of previously published Sr, Nd, and O isotopic data. Mass balance calculations show that no more than 50% of that mass can be mantle-derived. Flare-ups must have fundamentally developed simultaneously with crustal/lithospheric thickening, thus implying a connection. Subduction erosion from the trench side, and retroarc shortening from the foreland side are the main tectonic shortening processes that operate in conjunction with high flux magmatism during subduction, and therefore are likely triggers for flare-up events in arc. These arcs represent the sites of crustal differentiation, and thus contribute to net continental growth, only if dense residual lower crust was returned to the convective mantle. Isotopic data shown here suggest that if convective removal of batholithic roots takes place, it must be a consequence and not a cause of episodic flare-ups. The Altiplano-Puna Volcanic Complex in South America may be the most recent continental arc segment in flare-up mode. Keywords: subduction, isotopes, arc magmatism, flare-up, delamination. doi: 10.1130/G23898A.1

Magmatic and detrital zircon ages from the Rocas Verdes basin, a tectonically juxtaposed remnant of sea floor in the Magallanes fold and thrust belt (southern Patagonia, South America), indicates that a rifting phase of the Rocas Verdes basin occurred between 152 and 142 Ma, and was accompanied by bimodal magmatism. A dacite dyke cross-cutting pillow-basalt successions and a plagiogranite dyke in mixed mafic felsic terranes of the basal Sanniento Ophiolite Complex contain 150 Ma zircon crystals, indicating that Inafic submarine volcanism had started prior to or during the Late Jurassic, 10-15 Ma earlier than previously thought. The silicic pyroclastic rocks of the Toblfera Formation, with two samples dated at 148 and 142 Ma, were heralded by synrift sedimentation along fault-bounded grabens within Palaeozoic metasediments. No evidence for an active volcanic arc during the early formation of the Rocas Verdes basin was detected in detrital zircon grains of the lower sedimentary member of the Tobifera Formation. A minimum of 25 Ma of continuous sedimentation in the Rocas Verdes basin is suggested by detrital zircon grains in the upper member of the Zapata Formation. The Rocas Verdes basin was rimmed on the western side by an incipient and subaerial magmatic arc only in its later evolution.

It is widely held that a large composite terrane including the East Siberian shelf, Northern Chukotka and Arctic Alaska moved to its current location from the other side of the Mesozoic ocean in the course of the Amerasia basin opening. The western boundary of this terrane is critical for tectonic models of the Mesozoic Arctic. Consequently, the New Siberian Islands, which lie in the centre of this critical region, should help to define the position of this boundary. We have studied subvolcanic gabbroic intrusions on Bel'kov Island, the westernmost island in the New Siberian Archipelago. Their age is 252 [+ or -] 2 Ma (U-Pb zircon, secondary ion microprobe technique) and they represent mantle-derived tholeiitic magma modified by continental crust contamination. Their age and petrographic and geochemical features are identical to those of the Siberian traps, suggesting that in Early Mesozoic time the western New Siberian Islands were adjacent to northern Siberia (modem coordinates). This indicates that the large composite terrane that includes the western New Siberian Islands is not exotic to Siberia as is currently implied in some Arctic Mesozoic tectonic reconstructions.

Detrital modes for 524 deep-marine sand and sandstone samples recovered on circum-Pacific, Caribbean, and Mediterranean legs of the Deep Sea Drilling Project and the Ocean Drilling Program form the basis for an actualistic model for arc-related provenance. This model refines the Dickinson and Suczek (1979) and Dickinson and others (1983) models and can be used to interpret the provenance/tectonic history of ancient arc-related sedimentary sequences. Four provenance groups are defined using QFL, QmKP, LmLvLs, and LvfLvmiLvl ternary plots of site means: (1) intraoceanic arc and remnant arc, (2) continental arc, (3) triple junction, and (4) strike-slip-continental arc. Intraoceanic- and remnant-arc sands are poor in quartz (mean QFL%Q < 5) and rich in lithics (QFL%L > 75); they are predominantly composed of plagioclase feldspar and volcanic lithic fragments. Continental-arc sand can be more quartzofeldspathic than the intraoceanic- and remnant-arc sand (mean QFL%Q values as much as 10, mean QFL%F values as much as 65, and mean QmKP%Qm as much as 20) and has more variable lithic populations, with minor metamorphic and sedimentary components. The triple-junction and strike-slip-continental groups compositionally overlap; both are more quartzofeldspathic than the other groups and show highly variable lithic proportions, but the strike-slip-continental group is more quartzose. Modal compositions of the triple-junction group roughly correlate with the QFL transitional-arc field of Dickinson and others (1983), whereas the strike-slip-continental group approximately correlates with their dissected-arc field.