Minor Crustal Assimilation Research Articles

Pleistocene to recent evolution of Mocho-Choshuenco volcano during growth and retreat of the Patagonian Ice Sheet

Abstract Mocho-Choshuenco volcano (39.9°S, 72.0°W) produced ~75 explosive eruptions following retreat of the >1.5-km-thick Patagonian Ice Sheet associated with the local Last Glacial Maximum (LGM, from 35 to 18 ka). Here, we extend this record of volcanic evolution to include pre- and syn-LGM lavas that erupted during the Pleistocene. We establish a long-term chronology of magmatic and volcanic evolution and evaluate the relationship between volcanism and loading/unloading of the Patagonian Ice Sheet via twenty-four 40Ar/39Ar and two 3He age determinations integrated with stratigraphy and whole-rock compositions of lava flows and glass compositions of tephra. Our findings reveal that the edifice is much younger than previously thought and preserves 106 km3 of eruptive products, of which 50% were emplaced immediately following the end of the penultimate glaciation and 20% after the end of the LGM. A period of volcanic inactivity between 37 and 26 ka, when glaciers expanded, was followed by the eruption of incompatible element-rich basaltic andesites. Several of these syn-LGM lavas dated between 26 and 16 ka, which crop out at 1500–1700 m above sea level, show ice contact features that are consistent with emplacement against a 1400- to 1600-m-thick Patagonian Ice Sheet. Small volume dacitic eruptions and two explosive rhyolitic eruptions dominate the volcanic output from 18 to 8 ka, when the Patagonian Ice Sheet began to retreat rapidly. We hypothesize that increased lithostatic loading as the Patagonian Ice Sheet grew prohibited dike propagation, thus stalling the ascent of magma, promoting growth of at least three discrete magma reservoirs, and enhancing minor crustal assimilation to generate incompatible element-rich basaltic andesitic to dacitic magmas that erupted between 26 and 17 ka. From an adjacent reservoir, incompatible element-poor dacites erupted from 17 to 12 ka. These lava flows were followed by the caldera-forming eruption at 11.5 ka of 5.3 km3 of rhyolite from a deeper reservoir atop which a silicic melt lens had formed and expanded. Subsequent eruptions of oxidized dacitic magmas from the Choshuenco cone from 11.5 to 8 ka were followed by andesitic to dacitic eruptions at the more southerly Mocho cone, as well as small flank vent eruptions of basaltic andesite at 2.5 and 0.5 ka. This complex history reflects a multi-reservoir plumbing system beneath Mocho-Choshuenco, which is characterized by depths of magma storage, oxidation states, and trace element compositions that vary over short periods of time (<2 k.y.).

Metasomatized mantle under the Eastern Mexican Alkaline Province: evidence from the Oligocene Rincón Murillo Gabbro, Sierra de San Carlos-Cruillas, NE México

The Cenozoic Sierra de San Carlos-Cruillas (SSCC) magmatic complex belongs to the Eastern Mexican Alkaline Province (EMAP). The SSCC mainly consists of plutonic and subvolcanic bodies displaying diverse lithological compositions. These rocks exhibit both OIB- and arc-like geochemical characteristics. In this contribution, we present new petrographic, geochemical, isotopic, and geochronological data of the Rincón Murillo Gabbro (RMG), situated in the southern part of the SSCC. The RMG is nepheline normative and displays an OIB-like intraplate character. Laser ablation ICP-MS U-Pb analyses on titanites yielded Oligocene (Rupelian) crystallization ages of 31.8 ± 1.8 and 31.0 ± 1.7 Ma. The 87Sr/86Sr(i) (0.703400–0.704159) and 143Nd/144Nd(i) (0.512732–0.512815) ratios indicate a mantle origin. Trace element data indicate the participation of different enriched mantle components in the source. One component is related to previous subduction processes, generating the HFSE-depleted arc-like magmas of San José Gabbro in the northern part of SSCC. The second component is related to deep mantle melts with a slightly carbonatitic signal and HFSE-enriched fluids derived from the subducted Hess conjugate plateau generating different enriched sources in the upper mantle under the SSCC. The passage, eclogitization, detachment and foundering of the Hess plateau gave rise to an asthenospheric upwelling that triggered the low to moderate partial melting degrees (3–15%) of an amphibole-bearing garnet peridotite, generating the OIB-like parental magmas of RMG, located in the southern part of SSCC. Two-component mixing, and AFC models indicate that variable degrees of crystal fractionation and minor crustal assimilation are involved in the evolution of Rincón Murillo Gabbro.

Rapid transition to fertile magma and promotion of porphyry mineralization: A case study from the Don Javier deposit

The Don Javier porphyry Cu–Mo deposit formed contemporaneously with the Incaic I orogeny (∼60 Ma) in southern Peru. The causative dacite porphyry is hosted by the Yarabamba Superunit, which is the youngest batholithic unit in the Toquepala arc. In this study, we conducted elemental and isotopic analyses on samples from the dacite porphyry and Yarabamba Superunit in an effort to clarify the origins and formation mechanism of the deposit. Zircon U–Pb dating shows that the young part of the Yarabamba Superunit was emplaced at 65.4 ± 0.7 to 63.5 ± 0.8 Ma, and the causative dacite porphyry was emplaced at Don Javier between 59.9 ± 0.4 and 59.2 ± 1.1 Ma. The Yarabamba Superunit and dacite porphyry have similar εHf(t) and δ18O isotope values, ranging from –4.6 to +1.9 and 5.1‰ to 6.4‰, respectively. The Yarabamba Superunit has initial Sr and Nd isotope values of 0.70533–0.70579 and 0.51242–0.51244, respectively. Isotopic data suggest that the Yarabamba Superunit and dacite porphyry evolved from an isotopically homogeneous magma reservoir with minor crustal assimilation. However, the dacite porphyry has whole-rock and zircon Eu/Eu* values of 0.9 and 0.34, respectively, higher than those of the Yarabamba Superunit (0.6 and 0.13).Additionally, the dacite porphyry has significantly higher apatite S content (0.07 wt%) and XCl values (0.21) than those of the Yarabamba Superunit (apatite S: 0.01 wt%; XCl values: 0.15). The calculated magmatic oxidation state shows that the dacite porphyry has a significantly higher magmatic oxygen fugacity (ΔFMQ +1.1) than the Yarabamba Superunit (ΔFMQ –0.5).Together, the data suggest that the magmas formed the Yarabamba Superunit and causative dacite porphyry are characteristics of distinct tectonic regimes. The magma of the Yarabamba Superunit was generated during normal subduction and underwent low-pressure, H2O-poor, plagioclase-dominated fractionation while the magma of the dacite porphyry was generated during arc compression and underwent high-pressure, H2O-rich, amphibole-(garnet) dominated fractionation, which led to a high oxidation state and volatile-rich magma that promoted porphyry mineralization at Don Javier. This rapid elevation of the magmatic oxidation state and volatile content occurred synchronously with the Incaic I orogeny (∼60 Ma), which indicates that this orogeny had a significant impact on the formation of the very large–super-giant porphyry ore systems in southern Peru.

Origin and sources of recent continental alkaline volcanism in the Southwest Ankaratra volcanic field, Central Madagascar: Insights from major and trace element and Sr, Nd, Pb, Hf, Os isotopes

We conducted 40Ar/39Ar, major element, trace element and Sr, Nd, Pb, Hf, Os isotopic studies of basaltic lavas from the Southwest Ankaratra volcanic field, in order to assess the origin and sources of recent monogenetic volcanism in the central highlands of Madagascar. All samples in this study yielded ages less than 110 ka, providing further evidence for Pleistocene volcanism in the central highlands of Madagascar including the Southwest Ankaratra volcanic field. The Southwest Ankaratra magmas are characterized by significant chemical and isotopic heterogeneity across the volcanic field and within an individual eruptive center (Iavoko cinder cone), similar to that observed in other monogenetic volcanic fields. The data suggest two distinct geochemical groups within the Quaternary Ankaratra magmas: trachybasalts and basanites. All of the samples have trace element abundances and patterns similar to those observed in ocean island basalts (OIB). The trachybasalts are characterized by more evolved compositions, yet display lower trace element abundances, than the basanites. Furthermore, the trachybasalts are characterized by more radiogenic 87Sr/86Sr and lower 143Nd/144Nd than the basanites, and are displaced to slightly lower 206Pb/204Pb, but similar 207Pb/204Pb and 208Pb/204Pb, compared to the basanites.Samples from the Southwest Ankaratra volcanic field display suprachondritic 187Os/188Os values that can be explained by minor crustal assimilation, but their trace element and Sr-Nd-Pb-Hf isotopic characteristics are incompatible with crustal assimilation, and instead represent features of their mantle sources. Our data show that the magmas are compositionally distinct from those expected for subcontinental lithospheric mantle (SCLM)-derived basalts, and likely derive from an asthenospheric mantle source polluted by plume materials and delaminated SCLM. Chemical and isotopic studies of Pleistocene alkaline basalts from SW Ankaratra suggest the involvement of the African superplume C in their origins, and further suggest that superplume materials must be widely distributed in the upper mantle beneath the entire EARS from the Red Sea to Madagascar, both near to and distal from the low seismic velocity mantle regions.

Petrogenesis of Mesozoic giant dike swarms and geodynamical insights about EMI-Gough flavors in the Equatorial Atlantic Magmatic Province

The Rio Ceará-Mirim magmatism produced Mesozoic giant arcuate dike swarms during the initial stage of the West Gondwana breakup near the Equatorial Atlantic Ocean (NE Brazil), where they assemble tholeiites and subordinate intermediate rocks (SiO2 > 57 wt%). A multidata approach based on petrography, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb isotopes enables the discrimination of four geochemical groups (two high-Ti and two low-Ti). High-Ti groups are composed of (1) basalts and basaltic andesites with TiO2 generally higher than 2.0 wt%, MgO > ~3 wt% (named the HT group) and rich in incompatible elements and (2) (trachy)andesites and trachytes with lower TiO2 (<2.5 wt%; Mg < 3 wt%) interpreted as fractionated magmas from the HT group, therefore named evolved HT. Both these groups have similar 87Sr/86Sr(127) (~0.706) and εNd(127) (−2.90 to −2.56) and moderately radiogenic 206Pb/204Pb(m) ratios between 18.66 and 18.21. Conversely, low-Ti tholeiites (TiO2 < 2 wt%) are (3) LT1 composed of evolved basaltic andesites, with ~Ti/Zr < 70, MgO <6 wt%, and Al2O3 > 15 wt%; and (4) LT2 composed of subalkaline (to transitional) basalts and basaltic andesites but ~Ti/Zr >70, and MgO > 6 wt%. These two low-Ti groups are, respectively, less radiogenic (LT1 εNd(127) -4.65 to −4.40) and slightly more radiogenic (LT2 εNd(127) -1.44 to −1.08) in Nd relative to the broad HT group. The geochemical groups present different degrees of enrichment in large ion lithophile and light rare-earth elements coupled with depletion in high-field-strength elements (NbTa) combined with isotopic (Sr-Nd-Pb) signatures compatible with enriched mantle sources. We showed that the EMI (Gough-like) isotope compositions of Rio Ceará-Mirim magmatism may be explained by mixing DMM with enriched SCLM-derived melts plus minor crustal assimilation or by the involvement of OIBs derived from mantle plumes of the South Atlantic area mixed with lithospheric sources. In a geodynamical perspective, given the narrow similarity of HT Rio Ceará-Mirim isotope compositions with EMI-Gough type and high-Ti magmas of Transminas swarm, a genetic connection between Equatorial Atlantic and Paraná-Etendeka provinces must be considered.

The Oligocene Avaj volcanic – plutonic complex of Central Iran: A record of magma evolution and mineral equilibria

• High-An plagioclase feldspar cores formed during early stage of evolution of the primary magma . • Mineral-melt equilibrium conditions are determined in clinopyroxenes and plagioclase cores. • Zoning in plagioclase and smooth CSD patterns invoke an unsteady-state magma system. • Parental mafic magmas were hydrous and contained <4 wt% H 2 O. • The residence time of plutonic and volcanic magmas was 117 and 13 years, respectively. The Avaj Oligocene volcanic – plutonic complex is part of extensive Cenozoic magmatic activity within the Urumieh-Dokhtar magmatic arc of Iran. We use whole rock geochemistry, mineral compositions and crystal size distributions (CSD) in a suite of co-genetic basalt, basaltic andesite and gabbro to determine their petrogenesis. Ca-rich cores in plagioclase (An 79-86 ) overlap empirically modelled compositions, indicating equilibrium crystallization from melts represented by the whole-rock compositions. Clinopyroxene compositions (Mg# 74–80) are compatible with mildly fractionated mantle-derived magmas in an arc setting. Mineral-melt equilibrium is inferred from high Al contents and close correspondence between the measured DiHd and predicted Kd Fe–Mg (0.23–0.32) in clinopyroxenes, and Kd (Plg/melt) (An–Ab) values of plagioclase cores (0.11–0.15). Clinopyroxene-melt thermometers indicate crystallization at 1119–1173 °C for volcanic and 1099–1134 °C for plutonic rocks. Plagioclase crystal core saturation temperatures range from 1088–1162 °C (volcanic) and 1121–1163 °C (plutonic); these values overlap calculated mineral-melt equilibrium temperatures. Plagioclase CSDs are nearly straight for both volcanic and plutonic samples, with higher nucleation density and steeper slopes for the plutonic samples. Major element variations suggest the Avaj rocks represent co-genetic magmas related by fractional crystallization of the observed mineral phases. We suggest minor crustal assimilation occurred during ascent from a deeper reservoir to a shallower one; CSD data indicate longer magma residence time for plagioclase in the plutonic samples (∼117 years) compared to the residence time of basaltic samples (∼13 years).

In-situ U–Pb dating of zircon coronas, Sr–Nd–Hf isotopes and petrological constraints of the Daxigou anorthosite complex, NW China

Massif-type anorthosites, mainly Proterozoic in age, have long been recognized as a signature of crust-mantle interactions. The Daxigou Anorthosite Complex (DAC) is one such massif situated in the Kuluketage block, a tectonically important domain between the Tarim Craton and the Central Asian Orogenic Belt. In this contribution, a combination of geochronological and geochemical analyses of fine-grained DAC diabase are used to constrain the characteristics of the parental magma and the initiation time of DAC magmatism. We then pinpoint the DAC’s closure time by in-situ petrographic thin section U–Pb dating of zircon coronas around Fe-Ti oxides identified in a gabbroic anorthosite. Within data uncertainty, we find that the closing age of 1813 ± 9 Ma, is indistinguishable from the 1804 ± 7 Ma crystallization age of its last episode of parental magma. MELTS simulations of the parental diabase magma suggest a pressure of 8 kbar at a fayalite-magnetite-quartz oxygen fugacity leads to broadly observed mineral compositions and mineral phases. The bulk rock major and trace elements, integrated with the whole-rock Sr–Nd and Hf isotopes in zircon, indicate that the DAC was formed by partially melting of a metasomatized sub-continental lithospheric mantle, which was likely induced by a post-collisional slab break-off with minor crustal assimilation. Finally, we propose that the DAC magmatism can be ascribed to a series of Paleoproterozoic tectono-thermal events along the northern and southern margins of the Tarim Craton, which could have provided important implications for the Tarim Craton and the North China Craton associated with the assembly of Columbia supercontinent.

Tectonic evolution of the eastern Jiangnan region, South China: New findings and implications on the assembly of the Rodinia supercontinent

The Jiangnan orogenic belt in South China, formed by the collision of the Yangtze and Cathaysia sub-blocks, represents a key to reconstruct the assembly of the Rodinia supercontinent. Here we investigated the Fuchuan ophiolite mélange and Shexian magmatic units in the eastern Jiangnan belt, and report the first findings of anorthosite and leucogranite that are dated at 848 ± 4 and 840 ± 5 Ma, respectively. The anorthosites yield positive εNd(t) values (2.3–4.2) and lower initial 87Sr/86Sr ratios (0.704616–0.704679), along with zircon εHf(t) of ∼9.2, suggesting a depleted mantle contribution with minor crustal assimilation. The mafic-ultramafic rocks and anorthosite within the Fuchuan ophiolite display N-MORB & E-MORB signatures, likely generated in back-arc marginal sea spreading center and metasomatized by subduction-related fluids. The intermediate-felsic igneous suites from Shexian area in the eastern margin of the Zhanggong Terrane were formed at 850–830 Ma and display continental-arc-related geochemical features. Zircons within these rock suites mostly show εHf(t) values of 3–12, whereas inherited grains yielded negative εHf(t) values with model ages of 2.7–1.9 Ga, implying an active continental marginal environment. Overall relations and data indicate that the disrupted Fuchuan mafic-ultramafic rocks, anorthosite and leucogranite constitute the remnants of an 860–830 Ma ophiolite, whereas the northwestward subduction beneath the Zhanggong terrane occurred at ca. 870–860 Ma to ca. 830 Ma. A ca. 817 Ma muscovite-bearing K-granitic pluton dated at 817 ± 5 Ma emplaced in a post-collisional setting constrains the upper age limit of continental assembly along the Shaoxing- Jiangshan- Pingxiang- Qidong-Quanzhou- Longsheng-Yongfu fault zone. The timing of the final assembly of South China occurred at 830–800 Ma as referred from a collisional-type granitic magmatism ca. 830–815 Ma and an external dynamic process of uplifting-erosion-transpotation-deposition presented by the regional-scale unconformity with ages 820–795 Ma. Age and nature of the Jiangnan orogenic belt, along with sedimentary records and paleomagnetic constraints, indicate that the South China Block was possibly located along the northern Rodinia margin, adjacent to the Tarim and northwestern Australia blocks.

Mantle driven cretaceous flare-ups in Cordilleran arcs

Continental arcs display episodic magmatism characterized by flare-ups and lulls. Models published to explain these patterns invoke (1) upper plate crustal processes driven by internal feedback; (2) episodic mantle melting processes, or (3) external lower plate tectonic events.This study addresses the role of mantle magmas during flare-ups in Mesozoic Cretaceous continental arcs using geochronological and geochemical data for three Cretaceous arc segments: the western Peninsular Ranges Batholith (wPRB), the Peruvian Coastal Batholith (PCB), and the Chilean Coastal Batholith (CCB).In all three arc segments, bedrock zircon age patterns defining a flare-up from ~125 to 90 Ma characterized by gabbro to granite units with Sri < 0.705, ƐNd from 0 to +7.5, 208Pb/204Pb from 38.2 to 38.7, and 206Pb/204Pb from 18.3 to 18.7. These values project well towards a depleted mantle source. Areal measurements show that gabbro forms ~18% (wPRB), ~24% (PCB), and ~10% (CCB) of exposed plutonic material. AFC modeling indicates that these magmas have experienced fractional crystallization with only minor crustal assimilation (<20–30%), implying that the great majority of these magmas are mantle-derived. Thus the cause of these flare-ups must be episodic mantle processes: crustal melting was not required for triggering the flare-up, and only played a secondary role in modifying melt compositions. It remains unclear if the episodic mantle processes reflect internal feedback(s) or external tectonically driven processes.

What processes control the genesis of absarokite to shoshonite-banakite series in an intracontinental setting, as revealed by geochemical and Sr-Nd-Pb isotope data of Karadağ Stratovolcano in Central Anatolia, Turkey

The Latest Cenozoic absarokite to shoshonite-banakite series (ASB) samples were discovered in the Karadağ stratovolcano (KS) which erupted in the Karaman basin in Central Anatolia, Turkey; and has a spatially close association with major strike-slip faults and pull-apart basins. The KS are made up of adakitic andesite and dacite, with minor ASB lava flows. Here, we present integrated data of mineral chemical, whole rock major and trace element, Sr-Nd-Pb isotope and conventional KAr age of the ASB series rocks in Central Anatolia. The rocks were dated by conventional KAr dating method in old of 393 ± 153 ka. The series are comprised of slightly differentiated absarokite and more fractionated shoshonite-banakite lava flows. The Ol-normative absarokites contain phenocrysts of olivine and diopside, whereas the Hyp-normative shoshonites-banakites are made up of olivine, diopside, phlogopite, K-feldspar and plagioclase. Both series have a potassic nature and near-primitive character with high MgO (5–11 wt%) and Mg# (62 to 74). The overall samples were characterized relatively by the enrichment in large ion lithophile elements (LILE) and the depletion in high field strength elements (HFSE), with insignificant negative Eu anomaly. They possess homogeneous and unradiogenic Sr and Nd isotopic compositions, marked by 87Sr/86Sr(t) = 0.70507 to 0.70540 and εNd(t) =0.6 to +1.2. TDM, and Pb isotopic ratios tending towards the EMII mantle end-member.These geochemical features favor the origin of a lithospheric mantle reservoir which was metasomatized by i) sediments of subducting oceanic slab and ii) asthenospheric agents associated with break-off of African slab. The existing geochemical fingerprints and trace element modeling indicate that the Ol-normative absarokites were derived from a parental magma by low degree partial melting (~ %5) of a chemically enriched lowermost subcontinental lithospheric mantle source, composing of a garnet- and phlogopite-bearing lherzolitic lithology. The shoshonites-banakites were formed by fractionation of the absarokitic magmas, with a minor crustal assimilation. We contend that the Quaternary volcanism was probably controlled by the extension of the pull-apart basin system, which was induced by the deep strike-slip faults. Then, the extension caused an upwelling of the hot asthenosphere, triggering partial melting of the mantle source. This data rule out the possibility of the Latest Cenozoic active and steady subducting setting in the region. Instead, our results reveal the period which is likely marked by a transtensional phase and this phase triggered the pull-apart basin formation where the ASB series erupted in an intracontinental setting in Central Anatolia.

Petrological and geochemical characteristics of REE mineralization in the A-type French Creek Granite, New Zealand

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnasite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems.

Rhyolites in the Emeishan large igneous province (SW China) with implications for plume-related felsic magmatism

In the Emeishan Large Igneous Province (ELIP), western Yangtze Block, SW China, minor amounts of silicic volcanic rocks (∼5%) are spatially and temporally associated with flood basalts. The distribution and origin of these silicic rocks are keys to understanding the processes associated with mantle plume activities. In the Binchuan and Panzhihua areas, southwestern ELIP, there are Nb-Ta-poor and Nb-Ta-rich rhyolites. Nb-Ta-poor rhyolites interlayered with basalts in the Binchuan area occur in the lower part of the Emeishan volcanic sequence and have a crustal origin. They are characterized by negative Nb-Ta anomalies, low εNd(t) and high 87Sr/86Sr(i) values, and zircon saturation temperatures (836–852 °C). Nb-Ta-rich rhyolites are associated with the main phase of the Emeishan volcanic sequence in the Binchuan and Panzhihua areas and are characterized by positive Nb-Ta anomalies, oceanic island basalt (OIB)-like trace element ratios, low 87Sr/86Sr(i) ratios, positive εNd(t) and εHf(t) values, and high A/CNK ratios and Fe∗s. In addition, they have high Ga/Al ratios, high field strength elements (HFSEs), and high zircon saturation temperatures (>920 °C), typical of aluminous A-type silicic rocks, similar to the spatially distributed syenitic intrusions of the ELIP. These Nb-Ta-rich rhyolites are best interpreted as hybrid products of crystal fractionation of high-Ti basaltic magmas, coupled with minor crustal assimilation (6–14%), on the basis of Rhyolite-MELTS and EC-AFC simulations. The temporal and spatial distributions of these two types of rhyolite reflect various extents of thermal and mass exchange between mantle-derived basaltic magma and crustal material above a mantle plume. When the plume head reached the base of the western Yangtze Block, enriched components in the lithospheric mantle were melted and the melts partially ponded near the crust-mantle boundary or in the lower crust where they induced partial melting of crustal materials to form Nb-Ta-poor rhyolites. During the main phase of volcanism, large magma chambers and extensive plumbing systems developed due to decompressional melting of the mantle plume. In this stage, abundant mafic-ultramafic and felsic intrusions were formed, and Nb-Ta-rich rhyolites and A-type granites were emplaced.

Permian charnockites in the Pobeda area: Implications for Tarim mantle plume activity and HT metamorphism in the South Tien Shan range

The Permian history of the Central Asian Orogenic belt is marked by large-scale strike-slip faults that reactivate former Paleozoic structures, delineated by widespread alkaline magmatism. The genetic link between the syn-kinematic granitoids emplaced in the Tien Shan range and magmas emplaced within the Tarim Large Igneous Province, and the interaction between this plume and transcurrent tectonics, are still unsolved issues. We investigated the Pobeda massif, in the eastern Kyrgyz Tien Shan, located at the boundary between the Tien Shan range and the Tarim Craton, which exhibits a high-temperature unit. In this unit, Permian magmatism resulted in the emplacement of alkaline charnockites at mid-crustal levels. The primary mineralogical assemblage is nominally anhydrous and made of ortho- and clino-pyroxenes, fayalite, K-feldspar, plagioclase and quartz. These charnockites are associated with partially-molten paragneisses and marbles. Thermobarometry on these rocks indicates that the charnockites emplaced following the intrusion of a melt at a temperature > 1000 °C and pressure of around 6 kbar, corresponding to depth of ~20 km. The resulting thermal anomaly triggered the partial melting of paragneisses. Bulk geochemistry including Sr, Nd, Pb and Hf isotopes suggests that charnockites fit into the Tarim Large Igneous Province magmatic series, with minor crustal assimilation. U-Pb ages on zircons of charnockites and surrounding paragneisses indicate that charnockites intruded and triggered partial melting of the gneisses at c. 287, 275 and 265 Ma. 40Ar/39Ar dating on amphibole gives a similar age as the U-Pb age at 276.2 ± 2.0 Ma. 40Ar/39Ar dating on biotite from the Charnockite unit marbles gives ages at ca. 256–265 Ma, which shows that exhumation onset directly follows the HT history, and is tentatively correlated to top-to-the-North thrusting of the Charnockite unit in a transpressive context. Additional 40Ar/39Ar dating on syn-kinematic white micas from an adjacent transpressive shear-zone indicates continuation of the strike-slip tectonics at shallow crustal levels, after the exhumation of the Charnockite unit, at 248–257 Ma. These results demonstrate that Tien Shan Permian magmatism is linked to the Tarim mantle plume activity. Lithosphere-scale shear zones in the Tien Shan range, could have been responsible for lateral flow focusing of the Tarim mantle plume up to the boundary with the Tien Shan range and subsequent decompression melting resulting in the Permian magmatism observed in the Pobeda area.

Nature of the magma storage system beneath the Damavand volcano (N. Iran): An integrated study

Damavand intraplate stratovolcano constructed upon a moderately thick crust (58–67km) over the last 2Ma. The erupted products are dominantly trachyandesite-trachyte (TT) lavas and pyroclasts, with minor mafic magmas including tephrite-basanite-trachybasalt and alkali olivine basalts emplaced as cinder cones at the base of the stratovolcano. The TT products are characterized by a mineral assemblage of clinopyroxene (diopside-augite), orthopyroxene (clinoenstatite), feldspar (An2–58, Ab6–69, Or2–56), high Ti phlogopite, F-apatite, FeTi oxides, and minor amounts of olivine (Fo73–80), amphibole and zircon, whereas olivine (Fo78–88), high Mg# (80–89) diopside, feldspar, apatite and FeTi oxide occur in the mafic magmas. The presence of hydrous and anhydrous minerals, normal zonings, mafic cumulates, and the composition of magmatic inclusions in the TT products suggest evolutionary processes in polybaric conditions. In the same way, disequilibrium textures - including orthopyroxene mantled with clinopyroxene, reaction rim of phlogopite and amphibole, the coexistence of olivine and orthopyroxene, reverse, oscillatory and complex zonings of pyroxene and feldspar crystals - suggest magmatic evolutions in open systems with a varying temperature, oxygen fugacity, water as well as pressure and, to a lesser extent, melt chemistry. Mineral assemblages are used to model the physicochemical conditions and assess default parameters for the thermodynamic simulation of crystallization using MELTS software to track the P-T-H2O-ƒO2 evolution of the magma plumbing system. Thermobarometry and MELTS models estimated the initial nucleation depth at 16–17kb (56–60km) for olivine (Fo89) and high Al diopside crystals occurring in the mafic primary magma; it then stopped and underwent fractionation between 8 and 10kb (28–35km), corresponding with Moho depth, and continued to differentiate in the lower crust, in agreement with the geophysical models. The mafic rocks were formed by crystal fractionation from the reconstructed primary magma (13wt% MgO) with the minor role of recharge and crustal assimilation. Phenocrysts in TT lavas recorded a wide range of temperature and pressure of crystallization; at least three main levels of magma storage can be recognized according to the statistical analysis of the models, at 6–8kb (22–28km), 4–6kb (15–22km), and 0.6–3kb (2–11km), respectively. The temperature of crystallization ranged from 1430 to 1180°C for primary mafic magma to alkali olivine basalts and 1180–800°C for TT suite. According to the current geophysical models, the present structure of the magma storage system in the crust beneath the Damavand volcano consists of three major accumulation zones located at 20km, 6–8km and 3–4.5km depth. Our data enlarge this scenario, suggesting a more complex magma storage system strongly controlled by the transpressional tectonic regime. Multi-depth magma reservoirs may account for the local thickening of crust below the volcano. The polybaric fractionation model, using the MELTS algorithm, reproduces mineralogy and chemical variations of minerals and whole rock of the Damavand TT lavas. However, some discrepancies between major elements of models and trends of data can be ascribed to the recharge of more mafic magma, minor crustal assimilation, disaggregation of crystal-rich mushes and uptake of magmatic inclusions, as well as crystals from different crustal levels. The polybaric differentiation as the fractionation and/or accumulation of crystals was the probable mechanism for explaining the scarcity of mafic volcanic rocks at the Damavand volcano.

Geochemical and isotopic constraints on the role of juvenile crust and magma mixing in the UDMA magmatism, Iran: evidence from mafic microgranular enclaves and cogenetic granitoids in the Zafarghand igneous complex

The Zafarghand Igneous Complex is composed of granite, granodiorite, diorite, and gabbro that contain many mafic microgranular enclaves. This complex was emplaced during the late Oligocene (24.6 Ma) to form part of the Urumieh–Dokhtar magmatic arc of Central Iran. The enclaves have spheroidal to elongated/lenticular shapes and are quenched mafic melts in felsic host magma as evidenced by fine-grained sinuous margins and (or) locally transitional and diffuse contacts with the host rocks, as well as having disequilibrium textures. These textures including oscillatory zoning with resorption surfaces on plagioclase, feldspar megacrysts with poikilitic and anti-rapakivi textures, mafic clots, acicular apatites, and small lath-shaped plagioclase in larger plagioclase crystals all indicate that the enclaves crystallized from mafic magma that was injected into and mixing/mingling with the host felsic magma. The studied rocks have calc-alkaline, metaluminous compositions, with an arc affinity. They are enriched in large ion lithophile elements, light rare-earth elements, and depleted in high field strength elements with significant negative Eu anomalies. The Sr–Nd isotopic data for all of the samples are similar and display ISr = 0.705123–0.705950 and eNd (24.6 Ma) = − 1.04–1.03 with TDM ~ 0.9–1.1 Ga. The host granites and enclaves are of mixed/mingled origin and most probably formed by the interaction between the juvenile lower crust with a basaltic composition and old lower or middle continental crust as a major component and lithospheric mantle as a minor component; this was followed by fractional crystallization and possibly minor crustal assimilation. The source seems to be comprised of about 90–80% of the basaltic magma and about 10–20% of lower/middle-crust-derived magma. Geochemical characteristics indicate that the intrusion of these rocks from a subduction zone setting below the Central Iran micro-continent was related to an active continental margin, although was transitional to a transtensional setting possibly due oblique convergence to slab rollback or break-off.

Geochemistry of the Cretaceous Kaskanak Batholith and genesis of the Pebble porphyry Cu-Au-Mo deposit, Southwest Alaska

The key magmatic processes that lead to the formation of large magmatic-hydrothermal porphyry copper mineral deposits remain uncertain, and a particular question is why a few of these deposits, such as the Pebble porphyry Cu-Au-Mo deposit, are strongly enriched in both gold and molybdenum. This study investigated the igneous rocks of the Pebble district and obtained major and trace element compositions, Sr and Nd isotopic compositions, and zircon age and trace element data to model the origin of the ore-forming magmas. The Pebble porphyry Cu-Au-Mo deposit, one of the world’s largest Cu-Au resources, formed during the final stages of regional Late Cretaceous arc magmatism (101–88 Ma) in the Southwest Alaska Range. Local pre-mineral intrusions (99–95 Ma) are dominated by alkaline compositions including monzodiorite stocks, shoshonite dikes, and monzonite porphyries, but also include lesser volumes of high-K calc-alkaline diorite and granodiorite sills. The occurrence of early alkaline magmas has been noted at other gold-rich porphyry systems, including Bingham and Kerr-Sulfurets-Mitchell. Mineralization at Pebble is associated with granodiorite to granite porphyry dikes related to the >165 km 2 high-K calc-alkaline Kaskanak granodiorite batholith (91–89 Ma). Over a period of 10 m.y., Late Cretaceous melts evolved from high temperatures (930–730 °C) and modestly hydrous and oxidized conditions to relatively low temperatures (760–680 °C) and very hydrous and oxidized conditions. Collectively, all Late Cretaceous igneous rocks at Pebble contain magnetite and little or no ilmenite, are metaluminous to weakly peraluminous, and have typical arc trace element enrichments and depletions. They have moderate Sr/Y ratios (20–55) and gently sloped REE profiles (La/Yb = 5–20) that are not adakitic, which supports a source area lacking garnet that is consistent with a thin crust in southwest Alaska. Radiogenic isotopes for Late Cretaceous intrusions at Pebble have a restricted range of primitive Sr and Nd isotopic compositions ( 87 Sr/ 86 Sr i = 0.70329–0.70424; ɛNd i = 4.9–6.1), which overlap with volcanic and plutonic basement rocks of the Jurassic Talkeetna Arc along the Alaska Peninsula. The Kaskanak batholith intrudes the Late Jurassic–Early Cretaceous Kahiltna flysch, and mixing models using Sr and Nd isotopes indicate that the Kaskanak batholith assimilated ≤10 wt% Kahiltna flysch in amounts that did not likely affect magma fertility. Xenocrystic zircon samples are abundant in Cretaceous pre-mineral intrusions and have U-Pb ages similar to detrital zircon samples in the Kahiltna flysch. These data support some assimilation of upper crustal Kahiltna flysch, but the dominance of Devonian–Mississippian xenocrystic zircon populations in some intrusions suggests derivation from unexposed older basement. The extraordinary endowment of Cu and Au at Pebble is inferred to result from primitive calc-alkaline and alkaline arc magmas and the hydrous and strongly oxidized conditions that suppressed the formation and fractionation of Cu- and Au-enriched sulfide melts. Furthermore, differentiation to silicic compositions was a product of extensive crystal fractionation of parental melts accompanied by minor crustal assimilation. The trace element content of the intermediate composition intrusions indicates that both hornblende and titanite fractionation processes in the mid- to shallow-crust were both required to produce the more evolved granodiorite and granite porphyry compositions. Despite the apparent lack of Mo-enriched continental crust in the region, primitive hydrous melts were produced by protracted arc magmatism and were modified by minor crustal assimilation including early alkaline magmatism, periodic recharge of mafic hydrous basalts and hybrid andesites, and fractional crystallization, which was apparently sufficient to enrich Mo in late stage felsic melts.

Komatiites reveal a hydrous Archaean deep-mantle reservoir.

Archaean komatiites (ultramafic lavas) result from melting under extreme conditions of the Earth's mantle. Their chemical compositions evoke very high eruption temperatures, up to 1,600 degrees Celsius, which suggests even higher temperatures in their mantle source. This message is clouded, however, by uncertainty about the water content in komatiite magmas. One school of thought holds that komatiites were essentially dry and originated in mantle plumes while another argues that these magmas contained several per cent water, which drastically reduced their eruption temperature and links them to subduction processes. Here we report measurements of the content of water and other volatile components, and of major and trace elements in melt inclusions in exceptionally magnesian olivine (up to 94.5 mole per cent forsterite). This information provides direct estimates of the composition and crystallization temperature of the parental melts of Archaean komatiites. We show that the parental melt for 2.7-billion-year-old komatiites from the Abitibi greenstone belt in Canada contained 30 per cent magnesium oxide and 0.6 per cent water by weight, and was depleted in highly incompatible elements. This melt began to crystallize at around 1,530 degrees Celsius at shallow depth and under reducing conditions, and it evolved via fractional crystallization of olivine, accompanied by minor crustal assimilation. As its major- and trace-element composition and low oxygen fugacities are inconsistent with a subduction setting, we propose that its high H2O/Ce ratio (over 6,000) resulted from entrainment into the komatiite source of hydrous material from the mantle transition zone. These results confirm a plume origin for komatiites and high Archaean mantle temperatures, and evoke a hydrous reservoir in the deep mantle early in Earth's history.

New evidence of effusive and explosive volcanism in the Lower Carboniferous formations of the Moroccan Central Hercynian Massif: Geochemical data and geodynamic significance

The Azrou-Khénifra basin, located in the SE sector of the Moroccan Central Hercynian Massif of the Western Meseta of Morocco comprises volcanic and volcanoclastic rocks where two magmatic sequences can be distinguished: i) the Dhar Lahmar Sequence, composed of Upper Visean basaltic lava flows and pyroclastic deposits; and ii) the Kef Al Asri Sequence, composed of Visean – Serpukhovian intermediate to acid rocks.A continuous spatial and temporal evolution between the two volcano-sedimentary sequences was observed during the detailed geological work performed in the studied area. Petrography and geochemical studies additionally suggest a continuous compositional evolution from the more basic magmatic rocks to the intermediate/acid rocks, which implies a cogenetic magmatic differentiation controlled by crystal fractionation (with minor crustal assimilation) of a calc-alkaline trend magmatic suite. The inferred magmatic evolution is consistent with a geodynamic environment of an orogenic zone within an active continental margin setting.This partly explosive Visean – Serpukhovian volcanism, identified for the first time in the Western Meseta of Morocco, displays very similar petrographic and geochemical characteristics to its Eastern Meseta analogues, which implies that the emplacement of these magmatic rocks must have occurred in similar collisional geodynamic settings for both major geological domains, further constraining the evolution of this major crustal segment within the Carboniferous events that shaped the Hercynian Orogeny.

Late Cretaceous back‐arc extension and arc system evolution in the Gangdese area, southern Tibet: Geochronological, petrological, and Sr‐Nd‐Hf‐O isotopic evidence from Dagze diabases

AbstractBack‐arc extension and asthenosphere upwelling associated with oceanic lithospheric subduction affect the structure and thermal regime of the arc lithosphere, which often triggers widespread extension‐related mafic magmatism. Although it is commonly accepted that the Neo‐Tethyan oceanic lithosphere subducted beneath the southern Lhasa block, resulting in the well‐known Late Mesozoic Gangdese magmatic arc, the possible role of contemporary back‐arc extension and asthenosphere upwelling has been disputed due to a lack of evidence for extension‐related mafic magmatism. Here, we report detailed petrological, geochronological, geochemical, and Sr‐Nd‐Hf‐O isotopic data for the Dagze diabases located in the north of the Gangdese district, southern Lhasa block. The zircon U‐Pb analyses indicate that they were generated in the Late Cretaceous (ca. 92 Ma) instead of the Eocene (42–38 Ma) as previously believed. These mafic rocks are characterized by variable MgO (4.0–12.2 wt %) and Mg# (42 to 71) values combined with flat to slightly enriched ([La/Yb]N = 1.87–5.23) light rare earth elements (REEs) and relative flat heavy REEs ([Gd/Yb]N = 1.36–1.87) with negative Ta, Nb, and Ti anomalies (e.g., [Nb/La]PM = 0.16–0.51). They also have slightly variable εNd(t) (−1.25 to +4.71) and low initial 87Sr/86Sr (0.7045–0.7058) values with strong positive igneous zircon εHf(t) (+8.0 to +12.1) and low δ18O (5.31–6.12‰) values. The estimated primary melt compositions are similar to peridotite‐derived experimental melts. Given their high melting temperature (1332 to 1372°C) and hybrid geochemical characteristics, we propose that the Dagze mafic magmas likely represent mixtures of asthenospheric and enriched lithospheric mantle‐derived melts that underwent minor crustal assimilation and fractional crystallization of clinopyroxene. Taking into account the spatial and temporal distribution of Mesozoic mafic‐felsic magmatic rocks and regional paleomagnetic and basin data, we suggest that the Dagze mafic rocks resulted from asthenospheric upwelling associated with intracontinental back‐arc extension during the rollback of subducted Tethyan oceanic lithosphere in the Late Cretaceous.

A trachyte–syenite core within a basaltic nest: filtering of primitive injections by a multi-stage magma plumbing system (Oki-Dōzen, south-west Japan)

Oki-Dōzen (Japan) is a Late Miocene (7–5 Ma) intraplate alkalic volcano composed of a central trachytic pyroclastic complex surrounded by a ring-shaped succession of basaltic to trachybasaltic lavas and pyroclastic rocks and dispersed trachytic bodies. The central trachytic complex is in contact with a syenite that was intruded into the basement early Miocene volcano-sedimentary succession. In the centre of the system there are no alkalic basaltic rocks that are correlative of the outer flank ring. We present whole-rock major and trace element chemistry, Sr–Nd–Pb isotopic compositions and petrological data from the central trachytic volcanic complex and the intrusive syenite body, as well as from the outer ring basaltic succession. We also present and discuss a new set of zircon U/Pb ages collected from the central trachyte and syenite bodies. All the eruptive products of Oki-Dōzen, as well as the syenite, plot on a single liquid line of descent initiated from a mantle-derived alkalic basaltic parent. A younger (2.8 Ma) basaltic eruption (Uzuka basalt) has isotopic compositions that distinguish it from the rest of the system. Geochemical modelling indicates that magmatic differentiation through crystal fractionation and minor crustal assimilation occurred in crustal and shallow sub-volcanic magma reservoirs. In the central part of the system, a number of vertically spaced reservoirs acted as a filter, capturing basaltic dykes and hindering their ascent. In the outer region, dykes either reached the surface unhindered and erupted to form the basaltic/trachybasaltic succession or stalled at crustal levels and differentiated to trachyte before forming dispersed domes/flows. The central plumbing system “filter” resulted in a nest-shaped volcano, with a trachytic core surrounded by basaltic products, and stopped direct injection of basaltic magmas into the shallow syenitic magma reservoir, likely preventing its destabilization and explosive eruption.