Generation of Arc-Like and OIB-Like Magmas Triggered by Slab Detachment in the Eastern Mexican Alkaline Province: Petrological Evidence from the Cenozoic Sierra de San Carlos-Cruillas Complex, Tamaulipas
Luis Alejandro Elizondo-PachecoJuan Alonso Ramírez FernándezCristina de IgnacioReneé González‐GuzmánPedro Rodríguez-SaavedraVíctor Alejandro Leal‐CuellarFernando Velasco-TapiaJuan C. Montalvo‐Arrieta
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Abstract The origin of the Eastern Mexican Alkaline Province has been explained by landward arc migration and subsequent asthenospheric upwelling after slab roll-back of the Farallon Plate. In this work, we present new petrographic, mineral chemical, geochemical, and geochronological data of the Sierra de San Carlos-Cruillas (SSCC), one of the most important complexes in the province. This information, together with published data, helped us to reinterpret the tectonic processes operating during the generation of this province, as well as the mantle sources involved in the partial melting process. Detailed geochemical analysis suggests the participation of two types of metasomatized mantle regions: a lithospheric source modified by past subduction processes and an asthenospheric source slightly affected by carbonatite-related metasomatism. Variations in the partial melting degrees controlled the extent of magma enrichment in the latter. Major and trace element geochemistry, together with geochronological data and field relations, evinced an older post-orogenic setting related to the arc-like rocks (Eocene) and a younger intraplate extensional environment associated with all enriched igneous rocks (Oligocene-Miocene). Bivariate diagrams of SiO2-trace element ratios and multi-element patterns indicate that magmas from the SSCC complex dominantly evolved through fractional crystallization with a limited crustal contribution. Petrographic and mineral chemistry features suggest that some of these magmas experienced open-system processes (e.g. recharge events) in a complex and dynamic magmatic feeding system. In contrast to the traditional petrotectonic model, we propound that the passage and subsequent foundering of the Hess conjugate under northeastern Mexico resulted in its eclogitization and triggered slab tearing and succeeding detachment. This latter process occasioned mantle upwelling and the partial melting of the two recognized metasomatized mantle sources, thus generating the San Carlos-Cruillas magmatism.Keywords:
Metasomatism
Slab window
Fractional crystallization (geology)
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Petrogenesis
Continental arc
Metasomatism
Amphibole
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Incompatible element
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Carbonatite
Metasomatism
Fractional crystallization (geology)
Trace element
Incompatible element
Petrogenesis
Isotope Geochemistry
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Abstract The origin of the Eastern Mexican Alkaline Province has been explained by landward arc migration and subsequent asthenospheric upwelling after slab roll-back of the Farallon Plate. In this work, we present new petrographic, mineral chemical, geochemical, and geochronological data of the Sierra de San Carlos-Cruillas (SSCC), one of the most important complexes in the province. This information, together with published data, helped us to reinterpret the tectonic processes operating during the generation of this province, as well as the mantle sources involved in the partial melting process. Detailed geochemical analysis suggests the participation of two types of metasomatized mantle regions: a lithospheric source modified by past subduction processes and an asthenospheric source slightly affected by carbonatite-related metasomatism. Variations in the partial melting degrees controlled the extent of magma enrichment in the latter. Major and trace element geochemistry, together with geochronological data and field relations, evinced an older post-orogenic setting related to the arc-like rocks (Eocene) and a younger intraplate extensional environment associated with all enriched igneous rocks (Oligocene-Miocene). Bivariate diagrams of SiO2-trace element ratios and multi-element patterns indicate that magmas from the SSCC complex dominantly evolved through fractional crystallization with a limited crustal contribution. Petrographic and mineral chemistry features suggest that some of these magmas experienced open-system processes (e.g. recharge events) in a complex and dynamic magmatic feeding system. In contrast to the traditional petrotectonic model, we propound that the passage and subsequent foundering of the Hess conjugate under northeastern Mexico resulted in its eclogitization and triggered slab tearing and succeeding detachment. This latter process occasioned mantle upwelling and the partial melting of the two recognized metasomatized mantle sources, thus generating the San Carlos-Cruillas magmatism.
Metasomatism
Slab window
Fractional crystallization (geology)
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Petrogenesis
Continental arc
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Magmatic volatiles are critically important in the petrogenesis of igneous rocks but their inherent transience hampers the identification of their role in magmatic and metasomatic processes. For example, while the role of magmatic volatiles in porphyry copper systems is relatively well understood, the behavior of volatiles and metasomatic fluids in mantle and crustal magmatic-hydrothermal systems remains a work in progress. Alkaline-carbonatite complexes usually originate from the mantle and typically host REE deposits which are thought to be almost always of a hydrothermal origin. The question thus arises as to the origin of the REE and fluids and if the alkaline and carbonatite magmas are accompanied by fluids from the mantle up and through the crust. This study presents the results of whole rock trace elements and Sm–Nd isotopes analyses of clinopyroxenites, melteigites, ijolites, melanosyenites, leucosyenites, granites, silicocarbonatites, magnesiocarbonatites, calciocarbonatites, ferrocarbonatites and polygenic breccias from the 1894 Ma Paleoproterozoic alkaline-carbonatite complex of Montviel, Abitibi, Canada. The metasomatic rocks range in REE concentrations from 100 ppm to 3.54 wt.% and show εNd values ranging from + 6.8 to − 7.2. Based on these results it is demonstrated that volatile-saturated magmas at Montviel were injected through 4 distinct mantle pulses which evolved by fractional crystallization, mixing of depleted mantle and crustal fluids and metasomatism.
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The La Pedriza pluton stands out as the most extensively fractionated granite (Rb < 629; Sr < 2 and Ba < 2 ppm)
of the Spanish Central System Batholith. These granites show a strong enrichment in some rare metal contents
(Nb = 44, Y = 136, Yb = 10.7, U = 17, Ta = 7, Sc = 15 ppm). The petrography and geochemistry (including Sr-
Nd isotopes) reveal that the pluton is composed of at least four units. These are classified as I-type peraluminous
leucogranites (A/CNK=1.03-1.17), P-poor (P2O5<0.2 wt%) and Na2O-rich (< 4.24 wt%) exhibiting differences in
their HFSE and REE contents and eNd compositions. Moreover, the units of the La Pedriza granite display different
trends of fractional crystallization. REE spectra of the two most fractionated units suggest the involvement of a
fluorine-rich melt in the last stages favouring the crystallization of xenotime and niobotantalates. Intermediate
meta-igneous granulite protoliths are proposed as source rocks. The most evolved units of the La Pedriza pluton
show chemical features convergent to A-type granites; these are explained by extensive fractional crystallization
of a P-poor, I-type granite magma.
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We report major and trace element abundances for 147 samples and Sr, Nd, Hf, and Pb isotope compositions for a 36 sample subset of basaltic lava flows, sills, and dykes from the Karoo continental flood basalt (CFB) province in Botswana, Zimbabwe, and northern South Africa. Both low- and high-Ti (TiO2 < 2 wt % and > 2 wt %) rocks are included. MELTS modeling shows that these magmas evolved at low pressure (1 kbar) through fractional crystallization of gabbroic assemblages. Whereas both groups display enrichment in light rare earth elements (LREE) relative to heavy REE (HREE) and high field strength elements, and systematic negative Nb anomalies, they differ in terms of contrasting middle REE (MREE) to HREE fractionation, which is greater for the high-Ti basalts. This reflects different depths of melting of slightly enriched mantle sources: calculations suggest that the low-Ti basalts were generated by melting of a shallow spinel-bearing (2 % spinel) lherzolite, whereas the high-Ti magmas originated from a deeper-seated garnet-bearing (2–7% garnet) lherzolite. In most isotope plots, the high-Ti lavas together with the picrites define a common trend from Bulk Silicate Earth (BSE) to compositions with strongly negative ɛNdi and ɛHfi akin to those of some nephelinites and lamproites. The low-Ti rocks are shifted from BSE-like to more radiogenic Sr isotope ratios, indicative of upper crustal contamination. Trace element and isotope characteristics of the Karoo magmas require a combination of enrichment processes (subduction induced?) and long-term isolation of the mantle sources. We propose two distinct scenarios to explain the origin of the Karoo province. The first calls for polybaric melting of spatially heterogeneous, partially veined, sub-continental lithospheric mantle (SCLM). Calculations show that mixing between SCLM (∼BSE) and a strongly Nd–Hf unradiogenic nephelinite-like component (sediment input?) could account for the compositional variations of most of the high-Ti group lavas, whereas the mantle composition responsible for the low-Ti magmas is more likely to be similar to a vein-free, metasomatically enriched SCLM component. The second scenario involves mixing between two end-members represented by the SCLM and its deep-seated alkalic veins and a sub-lithospheric (asthenospheric- or ocean island basalt-like?) mantle plume. In this case, the data are compatible with an increasing mantle plume contribution as the plume rises and expands through the lithosphere. Regardless of which of the two scenarios is invoked, the spatial distribution of the low- and high-Ti magmas matches the relative positioning of the cratons and the Limpopo belt in such a way that strong control of the lithosphere on magma composition and distribution is a mandatory requirement of any petrogenetic model applied to the Karoo CFB.
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The La Pedriza pluton stands out as the most extensively fractionated granite (Rb < 629; Sr < 2 and Ba < 2 ppm) of the Spanish Central System Batholith. These granites show a strong enrichment in some rare metal contents (Nb = 44, Y = 136, Yb = 10.7, U = 17, Ta = 7, Sc = 15 ppm). The petrography and geochemistry (including SrNd isotopes) reveal that the pluton is composed of at least four units. These are classified as I-type peraluminous leucogranites (A/CNK=1.03-1.17), P-poor (P2O5<0.2 wt%) and Na2O-rich (< 4.24 wt%) exhibiting differences in their HFSE and REE contents and eNd compositions. Moreover, the units of the La Pedriza granite display different trends of fractional crystallization. REE spectra of the two most fractionated units suggest the involvement of a fluorine-rich melt in the last stages favouring the crystallization of xenotime and niobotantalates. Intermediate meta-igneous granulite protoliths are proposed as source rocks. The most evolved units of the La Pedriza pluton show chemical features convergent to A-type granites; these are explained by extensive fractional crystallization of a P-poor, I-type granite magma. 1
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Radiogenic nuclide
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Alkali basalt
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