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We develop a model of arc and behind-arc volcanism, as well as constraining the evolution of the lithospheric mantle structure beneath the Bolivian Andes during the last 25 Ma, using new geochronological and geochemical data on behind-arc mafic volcanic rocks, together with a regional isotopic study of geothermal helium emissions, and a comparison with seismic tomographic images of the same region. Helium isotopes measured in natural gas emissions in geothermal and mineral-water springs suggest recent mantle melting in a wide zone extending at c . 20°S from a few tens of kilometres west of the volcanic arc to over 300 km behind the arc, where the subducted slab is at depths increasing from c . 100 to c . 250 km. During the Neogene, there has been behind-arc mafic–felsic magmatism right across the high Andes between 17° and 22°S, in the Altiplano and western margin of the Eastern Cordillera. Modelling of Plio-Pleistocene basalts and basaltic andesites ( c . 15% depleted mid-ocean ridge basalt (MORB) source at depths between c . 75 km and c . 110 km for mafic volcanic rocks that crop out c . 65 km and c . 90 km for those that crop out >100 km behind the arc. REE inversions for Oligo-Miocene shoshonites and basalts ( c . 25–21 Ma) suggest that they originated at significantly shallower depths ( c . 45 km to c . 100 km) from a slightly enriched MORB source. Eruption of shoshonitic lavas also occurred in the Altiplano between 13 and 11 Ma, and widespread behind-arc intermediate–felsic volcanism has been more or less continuous since the early Miocene. All this suggests that since c . 25 Ma there has been a thin ( c . 25 Ma by the opening up of a mantle wedge and inflow of hot asthenosphere, as a consequence of the steepening of the subducted slab and detachment of hydrated and weakened mantle lithosphere and possibly mafic lower crust.

The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (206Pb/204Pb: 20.57, 207Pb/204Pb: 15.77, 87Sr/86Sr: 0.7030, eHf: + 3.8, eNd: + 4.2), compositions tending towards MORB (206Pb/204Pb: 19.01, 207Pb/204Pb: 15.62, 87Sr/86Sr: 0.7028, eHf: + 9.9, eNd: + 7.0) and compositions reflecting the influence of subducted sediments (206Pb/204Pb: 18.99, 207Pb/204Pb: 15.67, 87Sr/86Sr: 0.7037, eHf: + 4.4, eNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic 206Pb/204Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana.

The isotopic composition of helium emitted from geothermal springs in the southern Tibetan plateau, reported as Rc/RA (Rc=air corrected sample 3He/4He, RA=air 3He/4He), ranges from 0.013 to 0.38, and defines two principal domains. In southernmost central Tibet, helium isotope ratios are typical of radiogenic helium production in the crust (Rc/RA 0.1, mantle helium domain). The highest values of 0.27–0.38 RA occur at the southern end of the Karakoram fault. The boundary between the two domains lies 50–100 km north of the Indus-Zangpo suture zone. There seems to be no association between the 3He anomaly and zones of active normal faulting and litho-tectonic crustal units, such as the ultramafic rocks of the Indus-Zangpo suture zone and the Gangdese intrusive belt. Although scavenging of mantle-derived helium, stored in large ultrabasic and basic intrusions in the crust, cannot be ruled out entirely, we argue that the 3He anomaly most plausibly reflects degassing of volatiles from young (Quaternary) mantle-derived melts intruded into the crust. As such, it defines the southern limit of recent mantle melting and mantle melt extraction beneath the Tibetan plateau. The southern limit of the 3He anomaly coincides with the junction between the Indian and Asian plates, in the region where the Indian lithospheric slab steepens and is subducted beneath Tibet as suggested by seismic studies. Recent mantle melting and melt extraction is confined to the Asian mantle, but the southern limit of the melt zone may have migrated northwards during the last 10 Ma as the Indian lithosphere has progressively underthrust the Himalayas and Tibet.

Carbon stable-isotope compositions of coexisting carbon dioxide and methane from geothermal springs across the Central Andes of northern Chile and Bolivia are reported. A total of 60 samples were analyzed for δ13CCO2 and, of these, 10 were selected for δ13CCH4 analyses. The Central Andes are characterized by an active volcanic arc and an unusually thick (up to 75 km) continental crust behind the arc, beneath the high plateau region of the Altiplano. Furthermore, helium-isotope evidence suggests active mantle degassing in a 350-km-wide zone beneath the thick continental crust in the Central Andes (Hoke et al., 1994). The present results show a wide range of δ13CCO2 (-14.9 to -0.6‰) and a surprisingly heavy δ13CCH4 (−20.9 to −12.3‰). The difference between δ13CCO2 and δ13CCH4 (δ13CCO2-CH4 ) for individual samples varies between 1.5‰ and 13.5‰. The δ13CCO2 results show wide and overlapping ranges in the samples collected from the Precordillera, the Volcanic Arc (or Western Cordillera), the Altiplano, and the...

The Bolivian Altiplano, in the Central Andes of South America, is part of the second largest high plateau on Earth. It is an elongate region of subdued relief, ∼1.2 × 105 km2 and ∼4 km above sea level, bounded by the Eastern Cordillera and volcanic arc (Western Cordillera). Here the crust is up to ∼75 km thick. We describe the Cenozoic geological evolution of this region, using a revised chronostratigraphy and an analysis of the crustal and lithospheric structure. Crustal shortening and magmatic addition and, locally, sedimentation are the main mechanisms of Cenozoic crustal thickening, leading to nearly 4 km of surface uplift since the Paleocene. Addition of mafic melts appears to be a first-order mechanism of Cenozoic crustal growth, contributing ∼40% of the crustal thickening beneath the volcanic arc. Removal of the basal part of the lithosphere may have caused two episodes of widespread arc and behind-arc mafic volcanism, at ∼23 Ma and 0 – ∼5 Ma, contributing to the surface uplift. The Altiplano originated as a sedimentary basin, several hundred kilometers wide, between the proto-Western Cordillera and a narrow zone of uplift (proto-Eastern Cordillera) farther east. The latter zone formed by inversion of the center of a wide lacustrine or marine Cretaceous - Paleocene basin close to sea-level at ∼45 Ma. A thickness of 2–4 km of Paleogene continental elastics accumulated in the proto-Altiplano basin. Subsequently, in the Oligocene, we estimate that this region and the western margin of the Eastern Cordillera were technically shortened ∼22% (∼65 km), resulting in ∼9 km of average crustal thickening. The Altiplano basin was rejuvenated at ∼25 Ma and subsequently flooded with up to 8 km thickness of detritus eroded from the uplifting Eastern and Western Cordilleras. Between ∼25 and 5 Ma, folding and thrusting in the western margin of the Eastern Cordillera migrated westward into the center of the Altiplano basin, essentially terminating deposition, except in local subbasins, and accommodating ∼13% (∼30 km) of shortening and an estimated ∼7 km of average crustal thickening. Subsequently, there has been strike-slip deformation and limited local thrusting (< 5 km of shortening). Geomorphological and geochronological evidence for 1.5–2 km of surface uplift of this region since the Late Miocene suggests ∼14 km of lower crustal thickening beneath an essentially rigid “lid”, and can be explained by ∼100–150 km of underthrusting of the Brazilian shield and adjacent regions beneath the eastern margin of the Central Andes. The present subdued relief in the Altiplano may be a result of ductile flow in the lower crust and sedimentation and erosion in an internal drainage basin.