Sediment contribution to the mantle is the key step for the generation of orogenic magmatism to produce its isotopic and geochemical inventory. Even though they are exceptional for the post-collisional settings, there are worldwide examples of arc-related ultrapotassic mafic magmas which require complex multi-stage processes along with sediment melting e.g. in Italy or Pontides of Türkiye. To understand the metasomatism leading mantle to produce ultrapotassic mafic melts, we simulated the reactions of depleted (harzburgite) and fertile (lherzolite) mantle with subducted carbonate-rich sediment at relatively cold (800-850 °C) and shallow (2 GPa, 60-80 km) slab-mantle interfaces. The melting of sediments can trigger the formation of immiscible and conjugate carbonatitic and silicic melts which flux the mantle to develop hydrous minerals and dolomitic melt. The metasomatic growth product is a wehrlite composed of clinopyroxene, phlogopite, carbonate minerals and amphibole, representing a source of choice for Si-undersaturated ultrapotassic lavas. The occurrence of conjugate carbonatitic and silicic melts and their potential physical separation, offer a possibility for fractionation of several canonical trace element ratios such as Th/La, observed in Si-saturated ultrapotassic lavas. The synergy between peridotite-melt interaction and the physical separation of the carbonatitic and extremely K-enriched silicic melts are essential for the compositional evolution of ultrapotassic orogenic magmas and their mantle sources.
The analysis of tephra layers in maar lake sediments of the Eifel shows 14 well-visible tephra during the last glacial cycle from the Holocene to the Eemian (0–130,000 yr b2k). These tephra were analyzed for their petrographic composition, which allows us to connect several tephra to eruption sites. All tephra were dated by application of the ELSA-20 chronology, developed using the late Pleistocene infilled maar lake of Auel and the Holocene lake Holzmaar (0–60,000 yr b2k). We extend the ELSA-20 chronology with this paper for the millennia of 60,000–130,000 yr b2k (ELSA-23 chronology), which is based on the infilled maar lake records from Dehner, Hoher List, and Jungferweiher. The evaluation of the tephra from the entire last glacial cycle shows that all 14 tephra were close to interstadial warming of the North Atlantic sea surface temperatures. In particular, phreatomagmatic maar eruptions were systematically associated with Heinrich events or C-events. These events represent times of warming of the Southern Hemisphere, global sea level rise, and CO2 increase, which predate the abrupt interstadial warming events of the Northern Hemisphere. This synchroneity indicates a physical relationship between endogenic and exogenic processes. Changes in the lithospheric stress field in response to changes in continental ice loads have already been suggested as a potential candidate to explain the exogenic forcing of endogenic processes. The chronology of volcanic activity in the Eifel demonstrates that intraplate mantle plumes are also affected by the exogenic forcing of endogenic processes.
Abstract The Bin Yauri-Libata polymetallic ore district is a Sn and Au ore-bearing district in the Zuru schist belt, Northwestern Nigeria. The Libata Sn ore field is characterized by a set of cassiterite-bearing hydrothermal veins associated with Neoproterozoic Pan-African granites affected by deformation and low-grade metamorphism. The hydrothermal alteration associated with cassiterite-bearing quartz veins in the Libata deposit includes silicification, albitization, chloritization, and potassic alteration. In this study, geochemical and geochronological data from tourmalines and zircons from Sn-bearing lodes, unmineralized and altered granites is applied to reveal the timing, fluid composition, and source of ore-forming materials for tin mineralization in the Libata ore field. Zircon trace element and Hf isotopes [εHf(t) = +4.37 to +10.85] reveal a mantle-derived source with some crustal contribution for the melts forming the Libata Sn-bearing granites. LA-ICP-MS zircon U-Pb dating constrains the magmatic and hydrothermal ages to 650–646 and 649–646 Ma for the Libata granites. Overlapping zircon εHf(t) and 176Hf/177Hf but distinct 176Lu/177Hf and 176Yb/177Hf ratios from magmatic and hydrothermally altered zircons reveal a magmatic source for the hydrothermal fluids which triggered cassiterite deposition in the Libata ore field. Major element chemistry constrain tourmalines from the Libata ore field as schorls that show high alkalis, low-Ca contents, and moderate ☐ values (where ☐ is x-site vacancy). High Li, Zn, and Sn concentrations in tourmaline as well as Li/Sr and Ca-Fe-Mg ratios demonstrate that the tourmaline formed from granite-sourced fluid likely derived from the host Libata granites. Measured δ11B values from the Libata tourmaline range from –15.7‰ to –14.1‰. The δ11B of the mineralizing fluid is estimated to be –13.1 to –11.9‰ for the Libata tourmalines at 400–500 °C and overlaps with averages from fractionated granites worldwide. Therefore, our data show that tourmaline and zircon are useful tracers of magmatic-hydrothermal evolution in rare-metal-bearing granite systems.
The Sapat Complex in Northern Pakistan contains remnants of the northern Neo-Tethys Ocean, presently exposed along the Indus Suture Zone. The mantle peridotites of the Sapat Complex include harzburgites, dunites, and subordinate lherzolites. Harzburgites are the dominant peridotite variety over dunites. The dunites are hosted by harzburgites and occur exclusively as ‘envelopes’ surrounding chromitite pods. The podiform chromitites show disseminated, banded, and massive textures. Chromitites exhibit variable Cr# [Cr/(Cr + Al)] and Mg# [Mg/(Mg + Fe2+)], which range from 0.76 to 0.77, and from 0.64 to 0.66, respectively, while TiO2 contents are <0.2 wt.%. These features perhaps reflect crystallization of the chromian spinel from a boninitic magma. Similarly, chromian spinel in peridotites manifest a wide range of Cr# and Mg#, from 0.49 to 0.83 and 0.41 to 0.57, respectively, and are characterized by very low TiO2 values, averaging at 0.1 wt.%. Chromian spinel of chromitites and peridotites of the Sapat Complex have also very low Fe3+# (<0.01), which indicate their crystallization under low oxygen fugacities. The platinum group elements (PGE) distributions show high (Os + Ir + Ru)/(Rh + Pt + Pd), very low Pd/Ir values, and are defined by a prominently fractionated chondritic normalized PGE pattern, hence, this deposit is a typical example of an ophiolitic chromitite. The studied peridotites are highly depleted in PGE compared to chondritic values. The PdN/IrN values, averaging to 1.5 in dunites are unfractionated, while PGE spidergrams of harzburgites and lherzolites depict minor positive slopes, a minor positive Ru anomaly, and have average PdN/IrN values of 2.3 and 2.4, respectively. Furthermore, the harzburgites, dunites, and lherzolites display generally flat chondritic and primitive mantle normalized PGE patterns, and therefore, are nearly identical to highly depleted mantle peridotites. The mineralogical and PGE geochemical imprints of Sapat Complex chromitites and peridotites establish a strong affinity to supra-subduction zone ophiolites. Moreover, calculated parental melts of the chromitites and various geochemical discrimination diagrams elucidate that the chromitites were derived from boninitic magma produced by melting of depleted mantle peridotites in an oceanic arc, characterized by low oxygen fugacity, similar to a supra-subduction zone tectonic setting. This research highlights the use of mineralogy and geochemical compositions of chromitites and peridotites to reveal deep magmatic processes in a supra-subduction zone environment.
ABSTRACTThere is considerable debate regarding the petrogenesis of K-rich magma, which are believed to occur in subduction settings. The districts of the potassic rocks in the rear-arc of the Cenozoic Iran have not been thoroughly investigated. In this study, we investigate Late Eocene K-rich rocks of the Roshtkhar area of NE Iran. We employ whole-rock geochemistry, Sr-Nd-Hf-isotopes and zircon U-Pb age data of intrusive rocks to identify the mantle signature, evaluate the timing of emplacement, and determine the geodynamic setting. The Roshtkhar intrusive rocks (RIR) encompass a narrow compositional spectrum from syenite to monzonite, belonging to K-rich series, and are enriched in light rare earth elements and large ion lithophile elements such as Cs, Rb, Ba, Th, and U, with positive K, Pb, and Sr anomalies. U-Pb LA-ICP-MS dating of zircon from two samples yielded ages of ca. 38 Ma. Epsilon Hf values for investigated Eocene zircons display a variable range from +0.3 to +6.5. The RIR show relatively homogeneous initial epsilon Nd values ranging from +0.64 to −1.31 and modest variation in initial 87Sr/86Sr from 0.7048 to 0.7052. The data show mantle contribution with small involvement of Cadomian upper crustal material of up to 12% and an approximately 3% contribution fluid derived from subducted sediment into the mantle source. The lack of rocks that are purely derived from the lower crust in the study area further argues against the partial melting of lower crust that delaminated into the mantle. We suggest slab retreat, extension, and mantle upwelling as the main mechanisms for the generation of the primitive melts of RIR. Accordingly, decompression melting of metasomatized lithospheric mantle (e.g. phlogopite, apatite, and/or pargasite-bearing lherzolite) would be a plausible mechanism for the forming of K-rich basic melt in the mantle source.KEYWORDS: NE IranSr-Nd-Hf isotopesLate Eocene K-rich magmatismslab retreatextensionmantle upwelling AcknowledgmentsThe authors collectively express our gratitude to Dr. Hadi Shafaii Moghadam, Dr. Federico Lucci, and Dr. Gültekin Topuz for their constructive comments on the manuscript leading to substantial improvements of the paper. We thank Prof. Robert. J. Stern for his editorial comments and handling.Disclosure statementNo potential conflict of interest was reported by the author(s).Supplementary dataSupplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2185823Additional informationFundingThe work was supported by the evaluation model of resources potential for copper, nickel, and uranium in Western Asia.
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