Eocene Arc-related Magmatism within the Neotethys Subduction System in NW Iran; Geochemistry and SHRIMP zircon U-Pb Dating on Magmatic Phases in the Shahjahan Batholith

Granitoids of the composite Shahjahan batholith in the northernmost part of the Urmia-Dokhtar magmatic arc of Iran, and southernmost of the Lesser Caucasus (South Armenia) show SHRIMP zircon ages of 37.1±1.2 to 47.1±4.5 Ma. Dioritic rocks of the pluton with an age of 46.6 ± 4.6 to 47.1 ± 4.5 Ma are calk-alkaline to high-K calc-alkaline, metaluminous and I-type. They show arc-related affinities, characterized by LREE and LILE enrichment and HREE and HFSE depletion, especially negative Ti, Nb and Ta anomalies (TNT effect) in the normalized spider diagrams. low Ce/Pb, Nb/La and high Ba/Nb, U/Th and Hf/Zr ratios along with positive Pb, K, Th and Sr anomalies in the normalized spider diagrams for the studied samples are compatible with magma contamination with crustal materials during ascend to the lower crustal levels. Felsic dikes with granodiorite and syenite compositions and 37.1 ± 1.2 to 38.57 ± 0.41 Ma old, are characterized by high-K calc-alkaline to shoshonitic, metaluminous, and A2- type affinities which show post-collision tectonic setting geochemical features. The REE patterns for all studied samples and the composition of the trace element ratios indicate a geochemically enriched spinel-lherzolite lithospheric mantle source for the magmas, which underwent a low degree of partial melting. Dating arc-related dioritic samples and post collision felsic dikes put constrain on timing of Neotethys Ocean closure in NW Iran. Based on the present study, Middle to Upper Eocene is suggested as closure time of the Neotethys Ocean, Arabia and Central Iran plates’ collision and crustal thickening in Northwest Iran.

Reconciling the location of lava domes and eruption centers in Paleocene-Eocene calderas in northern Chile

<p>In the Atacama Desert, at the Precordillera of northern Chile, a series of Paleocene-Eocene caldera deposits and ring-faults are exceptionally well-preserved<sup>1</sup>. Here we aim to build on previous mapping efforts to consider the location, timing and style of pre, syn and post caldera volcanism in the region. We focus on the partially nested caldera complexes of Lomas Bayas and El Durazno<sup>2,3</sup> where deposits record several stages of caldera evolution (pre-collapse, collapse/intra-caldera and extra-caldera, resurgence and post-collapse eruptive deposits). The pre-caldera basement is a thick sequence of early Paleocene mafic lavas<sup>4, 5</sup>. The caldera complex formed between around 63 and 54 Ma<sup>4, 5</sup>. Both calderas constitute subcircular structures approximately 13 km in diameter and are cut by several NNW to NNE-trending felsic dikes which are spatially related to felsic domes interpreted as resulting from post caldera formation unrest<sup>1,</sup><sup>4</sup>. These calderas have been interpreted as part of the Carrizalillo megacaldera complex<sup>2 </sup>. We combine field observations, such as the attitude of dikes, as well as information on their dimension and composition, the size, location and composition of domes and lava flows, as well as the evidence of the regional stress field operating during the caldera evolution from measurements of fault kinematics. This data will be used as the input to finite element method models to investigate the effect of nested caldera geometry, ring-faults and crustal heterogeneities on the location of domes and eruptive centers generated during caldera unrest. The results will be potentially useful for constraining models of eruption forecasting during periods of unrest in calderas and ore deposition models which have been shown to be linked to caldera structure and magma emplacement.</p><p><strong>References</strong></p><p><sup>1 </sup>Rivera, O. and Falcón, M. (2000). Calderas tipo colapso-resurgentes del Terciario inferior en la Pre-Cordillera de la Región de Atacama: Emplazamiento de complejos volcano-plutónicos en las cuencas volcano-tectónicas extensionales Hornitos y Indio Muerto: IX Congreso Geológico Chileno, v. 2. Soc. Geol. de Chile, Puerto Varas.</p><p><sup>2 </sup>Rivera, O., and Mpodozis, C. (1994). La megacaldera Carrizalillo y sus calderas anidadas: Volcanismo sinextensional Cretácico Superior-Terciario inferior en la Precordillera de Copiapó, paper presented at VII Congreso Geológico Chileno. Acad. de Cienc. del Inst. Chilecol. de Geol. de Chile, Concepción.</p><p><sup>3 </sup>Rivera, O. (1992). El complejo volcano-plutónico Paleoceno-Eoceno del Cerro Durazno Alto: las calderas El Durazno y Lomas Bayas, Región de Atacama, Chile. Tesis Departamento de Geología, Universidad de Chile, 242. (Unpublished).</p><p><sup>4 </sup>Arévalo, C. (2005). Carta Los Loros, Región de Atacama. Servicio Nacional de Geología y Minería, Carta Geológica de Chile, 92, 1(100.000), 53 p.</p><p><sup>5 </sup>Iriarte, S., Arévalo, C., Mpodozis, C. (1999). Mapa Geológico de la Hoja La Guardia, Región de Atacama. Servicio Nacional de Geología y Minería. Mapas Geológicos, 13, 1(100.000).</p>

Geochemistry and Geochronology of the Neoproterozoic Backarc Basin Khzama Ophiolite (Anti-Atlas Mountains, Morocco): Tectonomagmatic Implications

The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 ± 5 Ma, which is consistent with the 761.1 + 1.9/−1.6 and 762 + 1/−2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica–low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 ± 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.

THE CASPOSO GOLD-SILVER DEPOSIT: EVIDENCE FOR PERMO-TRIASSIC LOW-SULFIDATION EPITHERMAL MINERALIZATION IN THE CORDILLERA FRONTAL, SAN JUAN PROVINCE, ARGENTINA

New geochronological data provide evidence for Permo-Triassic low-sulfidation epithermal gold-silver mineralization in the Cordillera Frontal, Argentina. The U-Pb sensitive high-resolution ion microprobe (SHRIMP) analyses on zircons and titanite gave the following results: (1) andesite and rhyolite volcanic host rocks of the Casposo Au-Ag deposit yielded a range of ages between 267.1 ± 0.7 and 241.7 ± 2.2 Ma; (2) two composite plutons located near Casposo yielded ages of 268.2 ± 1.5 and 265.1 ± 1.5 Ma for the Colorado syenogranite-granite pluton and 266.6 ± 1.4 and 254.0 ± 2.4 Ma for the Casposo granodiorite-tonalite pluton; (3) a trachyan-desite dike emplaced at 265.7 ± 1.2 Ma that is crosscut by mineralized quartz-adularia-calcite-gold veins in the Kamila East area; (4) felsite intrusions, interpreted to be temporally related to the emplacement of mineralized veins at 261.1 ± 3.5 Ma; and (5) composite rhyolite/andesite dikes that crosscut all other lithostratigraphic units and mineralized veins at 238.4 ± 1.6 Ma. The 40Ar/39Ar dates on hydrothermal adularia within quartz-adularia-calcite-gold veins of the Casposo deposit revealed at least three, likely discreet, hydrothermal fluid pulses and associated periods of vein formation during extensional events between 280–274, 262–258, and 250–246 Ma. Relative and absolute timing of volcanic host rocks, plutons, postmineralization felsic dikes, and gold-bearing veins of the Casposo epithermal vein system suggest the presence of significant Permian (Cisuralian)-Lower Triassic low-sulfidation epithermal-style gold-silver mineralization at the eastern flank of the Cordillera Principal in Argentina. The existence of this epithermal Au-Ag system opens the potential for a significant magmatic-hydrothermal system in a part of the Andes that previously was considered to be of low prospectivity.

Petrogenesis of Scheelite-Bearing Albitite as an Indicator for the Formation of a World-Class Scheelite Skarn Deposit: A Case Study of the Zhuxi Tungsten Deposit

Scheelite-bearing albitite is present in the form of rare, highly fractionated felsic dikes in the world-class Zhuxi tungsten deposit. Morphologically, the Zhuxi albitite forms individual dikes with thicknesses from 0.01 to 5.1 m in the orebodies. Additionally, the Zhuxi albitite is characterized by high sodium concentrations (Na2O = 6.08–8.04 wt %), low silicon (SiO2 = 56.81–62.56 wt %) and potassium concentrations (K2O = 1.44–2.62 wt %), and increasing P2O5 (0.1–0.7 wt %), Y (2.72–8.62 ppm), and rare earth element (8.28–28.89 ppm) concentrations from the tops to the bottoms of the dikes, which are controlled by the heterogeneous distribution of apatite grains in the albitite. The trace element geochemical characteristics and Sr-Nd isotope compositions of the albitite and the geochemistry of plagioclase, muscovite, apatite, and scheelite that formed in both the albitite and ore-related (altered) granites strongly suggest a genetic relationship between the two rocks. Given our new data and previous experimental data, as well as natural examples from around the world, we propose that the Zhuxi albitite is the product of a silicate-poor, H2O-rich melt that formed by melt–melt-liquid immiscibility processes in an extremely fractionated residual magma. A deep-seated (>3 kbar) granitic magma reservoir was directly related to the formation of these rare scheelite-bearing albitite dikes. Albitite dikes are the product of extreme fractionation of a granitic magma, and W is highly incompatible during magma evolution regardless of oxygen fugacity; therefore, intense tungsten mineralization development within albitite dikes should serve as an important criterion for judging the tungsten metallogenic potential.