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Magmatic processes in the continental crust such as crustal convection, melt ascent, magma emplacement, and batholith formation are not well understood. We solve the conservation equations for mass, momentum, and energy for two‐phase flow of melt and solid in 2D, for a thick continental crust heated from below by one or several heat pulses. A simplified binary melting model is incorporated. We systematically vary (a) the retention number, characterizing melt mobility, (b) the intensity of heat pulses applied at the bottom, and (c) the density of the solidified evolved rock. Two characteristic modes are identified: (a) in the “batholith emplacement mode,” segregation is sufficiently strong allowing melts to separate from the convective flow. This melt freezes to form buoyant SiO2‐rich layers. (b) In the “convective recycling mode,” melts are formed in the lower crust, rise together with the hot rock with little segregation, freeze at shallow depth but are partly recycled back to the lower crust where they remelt. Phase‐change‐driven convection dominates. Mode (a) is favored by high heat input, multiple heat pulses, high melt mobility, and low density of the evolved rock. Mode (b) is favored by less intense heating, less melt mobility, and denser evolved rocks. A scaling law is derived based on the thermal, melt, and compositional Rayleigh numbers and the retention number. The Altiplano‐Puna low‐velocity zone (LVZ) could represent the batholith emplacement mode with buoyant and voluminous magmas causing intense volcanism. The Tibetan LVZ is not associated with intense volcanism and might represent the convective recycling mode. Key Points Two‐phase flow models of crustal magmatic systems identify two modes: batholith emplacement versus convective recycling of evolved rock High melt mobility, multiple heating pulses, and low density of solidified evolved rock favor batholith emplacement The Altiplano‐Puna low‐velocity zone (LVZ) is in the batholith emplacement mode and the Tibetan LVZ is in the convective recycling mode

The Wooley Creek batholith is a Late Jurassic, arc-related, calc-alkaline plutonic complex in the Klamath Mountain province of California. Post-emplacement tilting and erosion have exposed ∼12 km of structural relief. The complex consists of an older (∼159.1 Ma) lower zone (pyroxenite to tonalite) assembled by piecemeal emplacement of many magma batches, a younger (∼158.2 Ma) upper zone (quartz diorite to granite), and a transitional central zone. In the lower zone, pyroxenes are too Fe rich to be in equilibrium with a melt whose composition was that of the host rock. Mass-balance calculations and simulations using rhyolite-MELTS indicate that these rocks are cumulates of pyroxenes and plagioclase ± olivine and accessory apatite and oxides. Percentages of interstitial melt varied from ∼7.5-83%. The plagioclase/pyroxene ratios of cumulates vary considerably among the most mafic rocks, but are relatively uniform among quartz diorite to tonalite. This near-constant ratio results in compositional trends that mimic a liquid line of descent. In the upper zone, bulk-rock compositional trends are consistent with differentiation of andesitic parental magmas. Upward gradation from quartz dioritic to granitic compositions, modeled via mass-balance calculations and rhyolite-MELTS simulations, indicate that structurally lower parts of the upper zone are cumulates of hornblende and plagioclase ± biotite and accessory minerals, with 37-80% trapped melt. In contrast, the structurally higher part of the upper zone represents differentiated magma that escaped the subjacent cumulates, representing differentiated melt fractions remaining from 92-54%. The ratio of cumulate plagioclase/(plagioclase + mafic minerals) ∼0.48 among upper-zone cumulates, mimicking a liquid line of descent. The results suggest that compositional variation in many calc-alkaline plutons may be at least as representative of crystal accumulation as of fractional crystallization. If so, then the assumption that arc plutons geochemically resemble frozen liquids is dubious and should be tested on a case-by-case basis. Moreover, comparisons of plutonic rock compositions with those of potentially comagmatic volcanic rocks will commonly yield spurious results unless accumulation in the plutons is accounted for.

Zircon U–Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California, was assembled over a period of at least 12 Ma between 96 and 84 Ma. Bulk mineral thermochronology (U–Pb zircon and titanite, 40 Ar/ 39 Ar hornblende and biotite) of rocks from multiple plutons comprising the Muir suite indicates rapid cooling through titanite and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Assembly of intrusive suites in the Sierra Nevada and elsewhere over millions of years favors growth by incremental intrusion. Estimated long-term pluton assembly rates for the John Muir Intrusive Suite are on the order of 0.001 km 3  a −1 which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions.

The Half Dome Granodiorite, Yosemite National Park, California, is recognized in the field by euhedral, fresh-looking, black hornblende phenocrysts up to 2 cm in length. This variety of granodiorite typifies intermediate-age hornblende-phyric units of Cretaceous nested plutonic suites in the Sierra Nevada batholith. Although only inclusions of feldspar are evident in hand samples, the phenocrysts are riddled with up to 50% inclusions of every major mineral found in the host granodiorite plus metamorphic minerals formed during cooling. Amphibole compositions within single phenocrysts vary from actinolite with less than 1 wt% Al2O3 to magnesiohornblende with over 8 wt%. Elemental zoning within the amphibole is highly irregular on the micrometer scale, showing patches and polygonal zones with dramatically different compositions separated by sharp to gradual transitions. The chemical compositions of entire phenocrysts are equivalent to hornblende plus a small proportion of biotite, suggesting that the non-biotite inclusions are the result of metamorphism of the phenocrysts. Backscattered electron imaging shows evidence of brecciation that may have been the result of volume changes as hornblende was converted to actinolite. Pressure calculations using the Al-in-hornblende barometer show unreasonably wide variations on the micrometer scale that cannot have been produced by temperature or pressure variations during crystallization. These hornblende phenocrysts would thus be unsuitable for geobarometry, and caution must be used to avoid similarly zoned phenocrysts in the application of the Al-in-hornblende geobarometer.

Thermometric properties and compositions of fluid inclusions in quartz are used to constrain the roles that fluid-soluble elements, principally Li, B, Cl, and F, have in controlling the transition from magmatic to hydrothermal mineral paragenesis in pegmatites and to ultimately understand why some pegmatites in the San Diego County pegmatite district contain abundant, gem-quality, Li-bearing minerals in pockets, whereas others do not. In this district, lithium-cesium-tantalum type pegmatites occur in the Mesozoic Peninsular Ranges Batholith. Emplacement of the dikes occurred at low pressures (200-300 MPa) that resulted in the formation of large miarolitic cavities (pockets), some of which contain gem-quality, Li-bearing minerals. Two pegmatite suites were studied: the gem-bearing Himalaya and the more barren La Posta. The inclusions measured in this study further underscore highly undercooled crystallization of pegmatites. Pressure-corrected homogenization temperatures (Th) of ∼400 to 515 and ∼70 to 425 °C were obtained for primary inclusions in the intermediate zone and the core, respectively, of a La Posta dike. Primary inclusions in the intermediate zone and the massive quartz core of the Himalaya pegmatite have Th ranges of ∼350 to 420 and ∼150 to 300 °C, respectively. The high portion of the latter temperature range is interpreted to represent the conditions that existed during the initial crystallization of minerals that line pegmatite pockets. The most important cations in fluid inclusions in both pegmatites are Na+, B3+, and Li+. Lithium concentrations are much higher in inclusions in the Himalaya pegmatite, up to 51 at% of all cations within the massive quartz in the core zone. In the La Posta pegmatite, few primary inclusions contain appreciable Li. The B content of inclusions in both pegmatites is high, up to 65 at% of cations. The dominant anions in the inclusions are Cl-, F-, and SO42-. The data suggest that hydrothermal fluids that collected in pockets were acidic and promoted the growth of tourmaline and other minerals that are stable in acidic solutions. In both pegmatites, Na and B dominate secondary inclusions. These inclusions reveal fluids stripped of Li and K by crystallization of lepidolite within fractures of primary minerals throughout the pegmatites, and sometimes as an alteration product in pockets. The lowering of alkali/H+ ratios in the fluid-stabilized clays, including kaolinite, that line the walls of pockets. Coeval crystallization of terminated quartz crystals with clays is consistent with its precipitation from the fluids.

New laser ablation inductively coupled plasma mass spectrometry U-Pb dating on zircon and monazite was performed to estimate the time required for the building of the Serre batholith in Calabria. Age spectra from the bottom and top of the pluton are characterized by two main peaks at 306 and 295 Ma, resulting from a mutual interference between serial intrusive events. On this basis, the emplacement of the top granodiorite layer postdates by about 10 m.yr. emplacement of the lower tonalite layer. These results have been incorporated into a two-dimensional numerical thermal model, assuming overaccretion of a batholith in an extensional tectonic regime. With this approach it was possible to reproduce pressure-temperature paths for various levels of the continental crust and define timing for low-pressure regional and contact metamorphism. In a unique tectonomagmatic scenario the model reproduces regional low-pressure metamorphic effects in the lower to intermediate continental crust and, with a time lag of about 6 m.yr., contact metamorphism in the upper crust. Finally, we propose a conceptual model for the emplacement of the Serre batholith in an extensional tectonic setting. Space for magma can be created by lower crust thinning and rock uplift at the bottom and top of the batholith, respectively.

This paper illustrates the geological and structural map (scale 1:10,000) of the Castellaccio Pluton (Asinara island - Italy), a peraluminous granodioritic intrusion of Variscan age that belongs to the Corsica-Sardinia Batholith. This small pluton, about 15 km 2 , emplaced within Paleozoic amphibolite-facies rocks during the post-collisional exhumation of the chain. The map shows the fine details of the magmatic structure of the pluton. Field-structural survey, coupled with remote sensing and major-elements geochemical mapping, allowed to recognize the occurrence of five magmatic units and the magmatic flow trajectories within the pluton. These outcomes provide useful insights into the architecture of the magmatic system. The results presented improve substantially the cartographic database of the Variscan crust of Sardinia and, finally, may serve as a robust background supporting further studies on the emplacement mechanism, or targeted to more specific petrologic studies.

The subsurface geometry of five representative late Archaean ‘Chilimanzi and Razi’ suite plutons in the Zimbabwe craton (ZC) has been investigated by gravity modelling constrained in part by surface geology, density measurements and seismic information, to determine their 3D configuration and infer tectonic context of emplacement. The generally K-rich, massive, homogeneous monzogranites are characterised by large Bouguer gravity lows (up to − 30 mGal amplitude) whose gradients outline their spatial extent well. The southernmost plutons and their anomalies have general trends paralleling the North Marginal Zone (NMZ) of the Limpopo orogenic belt (LB). Predictive gravity models indicate that the density contrast of the Chivi batholith (CB) adjacent to the ‘volcanic arc-like’ Belingwe greenstone belt extends to a depth of about 13 km. The nearby Razi pluton (RP) which intrudes the ZC-LB boundary appears to have been emplaced at shallower depths/levels. The gravity model suggests a thickness of about 5–6 km, and a moderate to shallow dip to the southeast under the NMZ, compatible with syn-kinematic intrusion during overthrust of the LB over the ZC. The smallest Nalatale granite (Ng) is on average 2.5 km thick under the Fort Rixon greenstone belt but includes a root up to 4.5 km thick under the anomaly peak, and a steep contact with the tonalite/gneiss to the east. These granites follow the general power-law for pluton dimension and are similar in this respect to the classical wedge-shaped plutons, extending largely in one direction, with large aspect ratios (Length (L)/Thickness (T) > 7). However, the overall shape of the RP is typical of a diapir (Width (W) < T), although it may have been affected by the LB deformation. Gravity modelling along a NS traverse crossing the Chilimanzi batholith (ChB), the Masvingo greenstone belt (MGB) and the Zimbabwe granite (ZG) indicate a thickness of around 6 km for the dense greenstone belt with a thickness of about 8.5 km for the adjacent ZG. The ‘complex’ shaped ChB shows a 2 km thick tabular body with a root zone extending to ~ 4.5 km depth on the south end, adjacent to the greenstone belt; typical of the so-called flat-floored plutons with a gently dipping floor towards the root zone. These two plutons roughly follow the power-law for laccolith/batholith dimensions (W/T > 5; L/T > 15). Overall, the CB and the ZG are interpreted as massive, deep-rooted batholithic intrusions (L/T ≅ 10), contrary to some geological interpretations of these late, post-kinematic intrusions as sheet-like bodies emplaced at relatively shallow levels in the crust. On the other hand, the ChB appears to be a tabular intrusion, probably fed by dykes; it exhibits a lateral extent much greater than the vertical one, outlining a sheeted geometry (W/T > 7; L/T > 18). The geophysical evidence, together with geological and fabric data, support and/or confirm the two main granite configurations: sheets and batholith; and thus also confirm the two main modes of emplacement: dyke and diapirism or ballooning plutonism. This is consistent with other known batholiths on the ZC but considered unusual for plutons of the same age and spatially close when compared to other Archaean cratons.

We present new zircon U-Pb isotopic data for volcanic rocks from deformed metavolcanic-sedimentary successions of the widespread Ross Supergroup in the Queen Maud Mountains, Antarctica. Zircon U-Pb analyses of Liv Group volcanic rocks thought to be Neoproterozoic in age instead have early Paleozoic ages. Zircon U-Pb analyses of five samples assigned to the Fairweather Formation have yielded 524±9 to 514±9 Ma (2σ) crystallization ages, whereas six samples assigned to the Taylor Formation have yielded 510±12 to 490±6 Ma (2σ) crystallization ages. Although these ages imply that the Fairweather Formation is generally older than the Taylor Formation, the age uncertainties show a 17-My overlap that is consistent with previous suggestions for temporal correlation of these formations. On a regional scale, Liv Group volcanism overlapped with the emplacement of ∼535-490 Ma plutonic rocks associated with the early Paleozoic Queen Maud batholith as well as igneous rocks found elsewhere along the early Paleozoic Pacific-Gondwana margin. Collectively, these igneous rocks provide plausible zircon sources for similar age detrital zircon populations found in outboard siliciclastic rocks belonging to the Leverett, Taylor, Fairweather, Greenlee, and Starshot Formations of the Queen Maud Mountains. The youngest crystallization age yielded by the deformed Taylor Formation (∼490 Ma) assumes regional significance because it represents the youngest volcanic rock yet identified within the Ross orogen in Antarctica and provides important new evidence for latest Cambrian or younger deformation, possibly associated with orogenic collapse during slab rollback at the terminal stages of the Ross orogeny.

Postcollisional extrusion and tectonic evolution in the eastern Tianshan orogenic belt (ETOB) remains poorly known, especially the mechanism of dextral strike-slip motion and associated tectonic exhumation. To better constrain this development, a structural and 40Ar/39Ar geochronological study was carried out on a syndextral strike-slip intrusion the Jueluotag batholith-as well as on other granitic plutons in the ETOB. 40Ar/39Ar analyses of hornblende, biotite, K-feldspar, and plagioclase from quartz-mica diorite, granodiorite, and dioritic porphyry dykes were used to construct cooling histories of the ETOB. Hornblendes have cooling ages of 277-272 Ma, similar to the syntectonic granitic intrusions, but biotite ages are 261-254 Ma along the syndextral strike-slip pluton from east to west. The dextral strike-slip motion cuts through ∼268-Ma dioritic porphyry dikes as well. From these data we conclude that dextral strike-slip motion occurred from ∼270 to 245 Ma. Based on the syntectonic granitic intrusions, structural features, and cooling ages along or outside of the dextral strike-slip belt, we demonstrate that a positive flower structure is the main structural framework for the Paleozoic northern segment of the ETOB. Rapid cooling and tectonic exhumation occurred during ∼240-220 Ma along the ETOB but did not occur in the western Tianshan orogen. The central Tianshan crystalline belt along the Gangou-Aqikekuduk fault zone was cut and offset southeastward by the dextral strike-slip motion. This suggests that dextral strike-slip motion occurred later than sinistral strike-slip along the southern margin of the ETOB. Geological features and age constraints suggest that the postcollisional eastward extrusion occurred at ∼270-245 Ma with dextral strike-slip motion, syntectonic granitic intrusions, and synextrusion tectonic exhumation.