The relationship, between magma generation and the tectonic evolution of orogens during subduction and subsequent collision requires self-consistent numerical modelling approaches predicting volumes and compositions of the produced magmatic rocks. Here, we use a 2D magmatic-thermomechanical numerical modelling procedure to analyse rapid subduction of a narrow ocean, followed by Mediterranean style collision, which is characterized by the gradual accretion of lower plate material and slab migration towards the orogenic foreland. Our results suggest that magmatism has a large-scale geodynamic effect by focusing deformation throughout the entire subduction and collision process. The rheological structure and compositional layering of the crust impose a key control on the distribution of magmatic rocks within the orogen. Compared to previous simplified homogeneous crustal models, a compositionally layered crust causes an increase in felsic material influx during continental collision and results in shallower magmatic sources that migrate with time towards the foreland. Changes in the deformation style may be locally driven by magma emplacement rather than by slab movement. Our modelling also demonstrates that the migration pattern of the deformation front and the magmatic arc relative to the location of the suture zone may be driven by lower crustal indentation in the overriding plate during early stages of collision. The modelling predicts a gradual change in magma source composition with time from typical calcalkaline to ones associated with relamination and eduction during subduction, collision and slab detachment. This transition explains the compositional changes of magma their temporal and spatial migration, as well as the observed link with deformation in the Dinarides orogen of Central Europe selected as a case study. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
There are many Miocene collision- and subduction-related porphyry Cu deposits in the Eastern Tethyan orogenic belt. However, the petrogenesis of the ore-related magmas and the mechanism of metal enrichment remain controversial. Here, we present a geochemical study and comparison of the major Miocene ore-related porphyries and coeval ore-barren magmatic rocks in the Urumieh-Dokhtar magmatic arc in Iran and the Chagai belt in Pakistan, and the Eastern Gangdese belt of the Himalaya. The results show that ore-related porphyries are characterized by relatively depleted mean Sr isotopic compositions (0.704217–0.706859 in the Urumieh-Dokhtar magmatic arc; 0.705052–0.706588 in the Eastern Gangdese belt; 0.705252–0.706708 in the Chagai belt), but higher maximum Sr/Y ratios (40–149 in Urumieh-Dokhtar; 79–178 in Eastern Gangdese; 52–178 in Chagai). In contrast, the coeval ore-barren magmatic rocks yield relatively enriched, mean Sr isotopic compositions (0.704722–0.707653 in Urumieh-Dokhtar; 0.704701–0.707802 in Eastern Gangdese; 0.705982–0.707856 in Chagai), but lower maximum Sr/Y (2–88 in Urumieh-Dokhtar; 61–136 in Eastern Gangdese; and 19–35 in Chagai). A deep crustal hot zone model provides a robust explanation of these data. Melts derived from a deeper melt source in juvenile mafic lower crust, where mantle-derived materials are common, have a greater potential to yield a higher tonnage of metallic Cu, such as the Sar-Cheshmeh deposit in Iran, which is derived from a deep melt zone, and characterized by a low radioactive mean Sr isotopic value of 0.704851, but a huge tonnage of metallic Cu of 7.2Mt and a high maximum Sr/Y value of 117 (representative of a deeper depth of the melt zone), whereas the Dalli deposit in Iran derived from a relatively shallow melt zone has a more radioactive mean Sr isotopic value of 0.706859, but a smaller amount of metallic Cu of 0.04Mt and a lower maximum Sr/Y ratio of 40. Conversely, re-melting the contact between newly-formed a
Cenozoic tectono-magmatic activities and metallogenic processes in the Kerman magmatic copper belt (KMCB) led to the formation of two contrasting porphyry copper systems in a post-collisional setting: (1) early to late Oligocene (i.e., 33.9-23.03 Ma) Jebal Barez-type intrusive rocks comprising small porphyry copper deposits (PCDs) of normal calc-alkaline and locally tholeiitic affinity, and (2) early Miocene to early Pliocene (i.e., 17-3.6 Ma) Kuh Panj-type granitoids hosting moderate to giant PCDs, such as the world-class Sarcheshmeh deposit with potassic and adakite-like calc-alkaline features. Here I present new Sr-Nd-Hf-O isotopic data from the both economic and sub-economic porphyries. The Kuh Panj ore-bearing granitoids exhibit low I-Sr (<0.706), high epsilon Nd(t) (>-4.6), positive zircon epsilon Hf(t) (>+6.1), low delta O-18 (<6.76%.), and T-DM(C) (Hf model ages) values of 425 to 705 Ma (mean T-DM(C) = 571 Ma). These isotopic data imply that the KMCB economic porphyry systems originated mainly from dehydration melting of a composite juvenile underplated thick lower crust derived from a depleted mantle source.
Detrital zircon geochronology is a powerful tool for investigating sedimentary provenance, but U-Pb zircon ages alone cannot distinguish between source terranes with similar age signatures. We integrate recent developments in petrochronology with sedimentary provenance analyses by investigating the relationship between tectonic stress regime and zircon Th/U. In the North American Cordillera, zircons that formed in melts associated with extensional magmatism contain variable Th/U (0.3 to >3.5), including significant zircon populations with Th/U >1.0, whereas zircons that crystallized from compressional magmatism exhibit low variability and low Th/U (<1.0). Higher temperature, more-fractionated, short-duration, bimodal magmatism in extensional magmatic systems may produce highly variable and elevated zircon Th/U. In compressional magmatic systems, lower temperature, long-lived, granitoid, oxidizing melts are more conducive to low Th/U zircon crystallization. Therefore, zircon Th/U may be correlated with end-member tectonic stress regimes (continental extension vs. continental arc) that correlate with distinct magmatic conditions.
An intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of subduction-related magmatism. Magmatism is most isotopically juvenile near the trench and becomes increasingly evolved landward. A compilation of radiogenic isotopic data from the central Andes, U.S. Cordillera, and Tibet (the most well-studied examples of modem and ancient Cordilleran systems) demonstrate such spatial trends are long-lived and persist throughout the life of these continental subduction margins. The consistency of the isotopic trend through time in magmatic products is surprising considering the plethora of orogenic processes that might be expected to alter them. In addition to longevity, spatial isotopic trends encompass a broad spectrum of geochemical compositions that represent diverse petrogenetic and geodynamic processes. The two end-members of the spatial isotopic trends are represented by melts sourced within isotopically juvenile asthenospheric mantle and melts sourced from isotopically evolved continental lithospheric mantle and/or lower crust. Mantle lithosphere generally thins toward the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes such as delamination, subduction erosion, and subduction ablation, operate to thin or remove the continental mantle lithosphere. With time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust, giving rise to an isotopically homogenous deep lithosphere. The results of this analysis have significant implications for interpreting temporal and spatial shifts in isotopic composition within Cordilleran orogens and suggest that the continental mantle lithosphere may be a significant source of magmatism in orogenic interiors. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
The Huimin metavolcanic rocks, consisting mainly of basalt, andesite, and dacite, are exposed predominantly in the Huimin area of the southern Lancangjiang zone, SW Yunnan. Three metavolcanic rock samples yielded zircon U-Pb ages of 456 +/- 3 Ma, 456 +/- 7 Ma and 459 +/- 14 Ma, suggesting that the eruption occurred in the Late Ordovician. These metavolcanics are characterized by sub-alkaline rocks, with high Al2O3 (12.45-17.68 wt.%) and low TiO2 (0.60-0.96 wt.%). The rocks are enriched in LREEs and LILEs with weakly negative Eu anomalies (Eu/Eu* = 037-0.99) and depleted in HFSEs, which is geochemically similar to typical subduction-related arc volcanic rocks. Zircons with Late Ordovician magmatic ages (similar to 456 Ma) have homogeneous Lu-Hf isotopic compositions with in-situ epsilon(Hf) (t) values of -4.36-3.30 and Hf model ages of 1.51-1.55 Ga, indicating that the Huimin metavolcanic rocks might have originated from a metasomatic mantle source. Our data provide reliable evidence for the presence of Ordovician magmatism in the Sanjiang area, represent the magmatic record of Prototethyan evolution in SW Yunnan, and identify a hitherto unknown early subduction event at the northeastern Gondwana margin. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Early Cretaceous magmatic rocks (including andesites, rhyolites, and granodiorites) occur in the Jiang Co and Zigetang Co areas of the middle segment of the Bangong-Nujiang suture zone, Tibet, and zircon U-Pb dating reveals that the magmatism took place between 118 and 113 Ma. The Zigetang Co andesites have geochemical features of the high-Mg adakitic andesites (HMAA) and are characterized by high K2O (2.95-3.58 wt%), Th (12.5-15.0 ppm), MgO (2.50-3.31 wt%), and Mg# (58-59), and relatively juvenile epsilon(Hf)(t) (+2.7 to +6.4) isotopic compositions. These observations suggest that the andesites were derived from partial melting of the delaminated juvenile lower continental crust of the northern Lhasa terrane. The Zigetang Co granodiorites represent the melts generated by high-degree fractional crystallization of the HMAA magma. The andesites from the Jiang Co area are interpreted as deriving from partial melting of ancient heterogeneous subduction-modified lithospheric mantle. The Jiang Co rhyolites are probably products of crustal anatexis in a heterogeneous source. Taking into account previous data, we propose that these late Early Cretaceous magmatic rocks developed in a postcollisional tectonic setting and that they were related to the delamination of thickened lithosphere following the final Lhasa-Qiangtang amalgamation. (C) 2016 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
U-Pb. Sm-Nd and Lu-Hf isotopic data for granitoid rocks from southern Malawi provide constraints on the timing and sources of magmatic activity within this segment of the Mozambique Belt and its role in the Rodinia and Gondwana supercontinent cycles. LA-ICP-MS single zircon U-Pb ages indicate a number of periods of magmatic activity: late Mesoproterozoic at ca. 1130 Ma, 1070 Ma, and 1050 to 1030 Ma: Neoproterozoic at ca. 960 Ma and 600 Ma: Cambrian at ca. 530 and 515 Ma, and Mesozoic at ca. 120 Ma. The oldest igneous activity, 1128 30 Ma, corresponds with emplacement of a chamockitic granitoid in the southeast corner of Malawi (Mulanje area). This region subsequently experienced metamorphism dated at 515 18 Ma. The youngest magmatism is alkaline in affinity and is associated with the East African Rift. Radiogenic isotope data indicate that the Mesoproterozoic samples have positive cNd and &If values, signifying derivation from material with a suprachondritic signature, whereas the younger rocks have negative values suggestive of crustal material recycling and mixing for their source and origins. The data imply that in the Malawi region of the Mozambique Belt, addition of new crust occurred during Rodinia assembly whereas magmatic activity during Gondwana assembly was restricted to reworking and mixing. (C) 2018 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
Post-collisional ultrapotassic magmatic rocks (15.2-18.8 Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into highMgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo(87-92)), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo(77-89)). These ultrapotassic magmatic rocks have high MgO (4.6-14.5 wt%), Ni (145-346 ppm), Cr (289-610 ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (Sr-87/Sr-86)(i) (0.7167-0.7274) and unradiogenic (Nd-143/Nd-144)(i) (0.5118-0.5120), high (Pb-207/Pb-204)(i) (15.740-15.816) and (Pb-208/Pb-204)(i) (39.661-39.827) at a given (Pb-206/Pb-204)(i) (18.363-18.790) with high delta O-18 values (7.3-9.7 parts per thousand). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7-14.5 wt%) subgroup displays higher (143Nd/144Nd) i, lower (87Sr/86Sr) i and (206Pb/204Pb) i ratios and lower delta O-18 values compared with the low-MgO (4.6-8.8 wt%) subgroup. High Ni (850-4862 ppm) contents of olivine phenocrysts and high whole-rock SiO2, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO2, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45-65 vol%) and orthopyroxene (30-50 vol%) with minor phlogopite (<3 vol%) and quartz (<2 vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60-75 vol%), orthopyroxene (20-35 vol%), clinopyroxene (<3 vol%), phlogopite (<2 vol%) and spinel (<2 vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25-8 Ma in south Tibet.