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<jats:p>The timing of deformation within and adjacent to the Helena salient of west-central Montana is poorly constrained relative to other segments of the Sevier fold-and-thrust belt. This study presents low-temperature thermochronology data from the Little Belt Mountains, a basement-cored Laramide uplift that is juxtaposed with the leading edge of the salient. We analyzed eight samples of Paleoproterozoic basement for apatite fission-track (AFT) and zircon (U-Th)/He (ZHe) thermochronology. Four samples yielded AFT ages ranging from ca. 80 Ma to 73 Ma and associated long, unimodal confined track lengths, indicating rapid cooling and exhumation of Little Belt Mountains basement during the Late Cretaceous. The other four samples are characterized by younger AFT ages (ca. 55 Ma), which suggest a combination of prolonged residence in the apatite partial annealing zone and postexhumation magmatic reheating. In total, 20 new ZHe dates range from ca. 236 Ma to 28 Ma and show a correlation between date and effective uranium. Forward model results for ZHe data are consistent with upper-crustal residence during the Proterozoic followed by Phanerozoic burial and rapid Late Cretaceous cooling. Cross sections across the Little Belt Mountains display the geometry of the Volcano Valley fault zone, an array of down-to-the-south Proterozoic normal faults that profoundly influenced the development of the Cordilleran thrust belt. Our new constraints from the Little Belt Mountains when integrated with published kinematic constraints from the Helena salient reveal significant out-of-sequence deformation in this portion of the thrust belt between ca. 80 Ma and 55 Ma. A kinematic model is proposed that involves Late Cretaceous (ca. 80 Ma) exploitation of rheologically incompetent units at the base of the Belt Supergroup within the Helena Embayment, facilitating early exhumation in the Little Belt Mountains. Our new data and synthesis are consistent with previous interpretations in which an inherited stratigraphic and structural architecture of Proterozoic ancestry was the predominant control on the development of the Helena salient during Cretaceous−early Eocene time.</jats:p>

<jats:p>The Marinoan diamictites and the overlying cap carbonates at continental margins bear key information on the paleo-environment evolution during the collapse of Snowball Earth, such as the timespan of intense chemical weathering. Such a sedimentary suite has been recently discovered in the northwestern Tarim Craton of China, but its depositional processes remain controversial. Here, we present stratigraphic and isotope geochemical studies on the diamictites of the Yuermeinak Formation and the overlying cap carbonates of the Sugetbrak Formation in the Aksu region of the northwestern Tarim Craton. Multiple unconformities in the region suggest major tectonic uplifting during the Cryogenian, probably resulting in a mountainous topography and varying dip directions of the overlying cap carbonates. The paleo-elevation of these mountains might have been higher and above sea level. We propose new depositional processes that involved four stages from glacial continental facies to neritic facies and/or alluvial fan systems. The first stage formed the massive diamictites and stratified siltstones with dropstones, recording cycles between glacier retreat and advance. The second stage involved the late transgression at the end of the deglaciation and the formation of calcareous massive diamictites with negative δ13C. The third stage included the onset of cap carbonate deposition and the alternating precipitation of calcareous mudstones and carbonates, reflecting frequent sea-level changes. The fourth stage was related to a widespread marine regression that developed a terrestrial environment and the sedimentation of the sandstones of the Sugetbrak Formation. Furthermore, we suggest that intense chemical weathering on exposed continents after the Marinoan deglaciation likely lasted for only hundreds of thousands of years, releasing ample alkalis into the ocean and facilitating the precipitation of the cap carbonates.</jats:p>

<jats:p>Clinochlore is a major hydrous mineral in subduction zones, and its thermophysical properties at high temperature and pressure are critical to the thermal structures of subduction zones. Here, we used the pulse heating method to measure thermal diffusivity and thermal conductivity of clinochlore at 0.5−4.0 GPa and 298−1373 K. Our results indicate that upon heating, thermal diffusivity and thermal conductivity decrease from ∼9.6 × 10−7 m2 s−1 to 4.3 × 10−7 m2 s−1 and from ∼3.5 W m−1 K−1 to 1.9 W m−1 K−1, respectively, before dehydration, but this trend is reversed after dehydration. In general, the pressure derivatives for the thermal transport properties also decrease with temperature before dehydration. Lattice heat transfer is the dominant mechanism before dehydration, but fluid is involved after dehydration. Using our experimental data, we simulated the temperature distribution of subducting slabs containing clinochlore at volume fractions of 0%, 10%, 20%, 50%, and 100%. Our simulations showed that the heat insulation effect caused by the presence of clinochlore could result in an increase in temperature by 30−60 K for the upper part of the subducting slab.</jats:p>

<jats:p>To understand the orogenic cyclicity that built the northern Yili magmatic arc in the southern Balhkash-Yili arc of the Central Asian Orogenic Belt, we synthesize its tectono-magmatic processes using new and existing U-Pb geochronological dates (228), Hf-in-zircon (1605 spots), bulk-rock geochemical data (1458), and Sr-Nd isotopic analyses (461) of magmatic rocks. Based on temporal-spatial distribution of magmatic rocks and their geochemical composition, four magmatic episodes (ca. 470−440 Ma, 420−380 Ma, 350−320 Ma, and 300−260 Ma) punctuated by a magmatic gap (ca. 440−420 Ma) and two high-flux magmatic pulses (ca. 380−350 Ma and 320−300 Ma) were recognized. These episodic magmatic processes are proposed to be dynamically associated with changes in configuration of the subducted Junggar oceanic slab, including 470−440 Ma initial subduction, 440−420 Ma flat subduction, 420−380 Ma retreating subduction, 380−350 Ma advancing subduction, and 350−320 Ma retreating subduction, followed by slab break-off in ca. 320−300 Ma and subsequent lithospheric delamination in a post-collisional setting during the Permian (ca. 300−260 Ma). In addition, tectonic transition (ca. 380 Ma and 320 Ma) from extension to compression were suggested to be closely related to the two phases of porphyry Cu mineralization events in the Balkhash-Yili arc. High crustal thickness and more mature arc evolution with prominent crust-mantle interaction may be responsible for the significant porphyry Cu endowment in the northern Balkhash-Yili arc.</jats:p>

<jats:p>The Miocene Kaiserstuhl volcanic complex in the Rhine graben rift is known for simultaneously exposing both intrusive and erupted (pyroclastic) calciocarbonatites. This makes Kaiserstuhl a promising candidate for studying the field and genetic relations between intrusive calciocarbonatite and its eruptive equivalent, and the processes enabling eruption of the calciocarbonatite at the surface in particular. Eruptive calciocarbonatites in Kaiserstuhl are represented by carbonatite tuff and lapillistone beds covering a debrite fan on the western flank of the volcano. The debrites are interpreted as lahar (debris flow) and possibly also debris-avalanche deposits. Based on the observed textures, the debris flows were most likely derived by water dilution from debris avalanches resulting from edifice failure, which occurred in the central part of the Kaiserstuhl volcanic complex. The edifice failure ultimately exposed the intrusive system, and the carbonatite pyroclasts (lapilli and ash) were ejected from narrow vents represented by open-framework tuff-breccias aligned along the detachment scarp. Since the Ca-carbonates break down rapidly at high temperatures and low pressures, calciocarbonatites are unlikely to form surface lavas. On the other hand, the presence of the calciocarbonatite pyroclastic deposits suggests that some geological process faster than the high-temperature breakdown of Ca-carbonate may facilitate calciocarbonatite eruption. We suggest that the sudden exposure and decompression of a suprasolidus high-level carbonatite intrusion by edifice collapse may be a suitable scenario enabling calciocarbonatite eruption. The absence of edifice failures on alkaline volcanoes, where carbonatite intrusion is either supposed or exposed, may explain the overall scarcity of erupted calciocarbonatites.</jats:p>

<jats:p>The Upper Triassic Popo Agie Formation of the Chugwater Group of Wyoming, northeastern Utah, and northwestern Colorado has been an enigmatic unit since its definition and is commonly excluded from large-scale studies of continental Upper Triassic strata in the western USA. Lithostratigraphic correlations of Popo Agie Formation outcrops are documented from west-central Wyoming through northeastern Utah and northwestern Colorado, which demonstrates the presence of the Popo Agie Formation throughout this region. Unique detrital zircon age distributions have led previous workers to hypothesize a paleodrainage connecting basal units of the Dockum in west Texas and eastern New Mexico, USA, with the Gartra Grit, a basal unit of the Popo Agie in northeastern Utah. Biostratigraphically informative taxa such as parasuchid phytosaurs in the absence of leptosuchmorph phytosaurs support an assignment of the Gartra Grit and Popo Agie Formation to the Otischalkian estimated holochronozone. We present the first detrital zircon age distributions for the Popo Agie Formation. Multidimensional scaling analysis of zircon populations shows that the Popo Agie samples are similar to other Upper Triassic units surrounding the Ancestral Uncompahgre Highlands, Central Colorado Trough, and the Ancestral Front Range. Additionally, we present the first maximum depositional ages (youngest population) for the Popo Agie Formation at a location where the top of the ochre unit is no older than 225 ± 4 Ma, and the upper purple to ochre transition is no older than 230 ± 5 Ma. By leveraging existing biostratigraphy, regional lithostratigraphy, and new radioisotopic ages we temporally constrain the Popo Agie Formation, enabling us to integrate the upper Chugwater Group, Chinle Formation, and Dockum Group strata into a testable Late Triassic chronostratigraphic framework for the western USA. The consilience of data support a Carnian age for the majority (if not entirety) of the Popo Agie Formation, making this—and equivalent strata in the Dockum Group (i.e., Camp Springs Conglomerate, and strata of the Tecolotito and Los Esteros members of the Santa Rosa Formation)—among the oldest continental Late Triassic stratigraphic units in the western USA.</jats:p>

<jats:p>Volcanic hazards associated with lava flows advancing on snow cover are often underrated, although sudden explosions related to different processes of lava-snow/ice contact can occur rapidly and are only preceded by small, easily underrated precursors. On 16 March 2017, during a mildly effusive and explosive eruption at Mount Etna, Italy, a slowly advancing lava lobe interacted with the snow cover to produce a sudden, brief sequence of explosions. White vapor, brown ash, and coarse material were suddenly ejected, and the products struck a group of people, injuring some of them. The proximal deposit formed a continuous mantle of ash, lapilli, and decimeter-sized bombs, while the ballistic material travelled up to 200 m from the lava edge. The deposit was estimated to have a mass of 7.1 ± 0.8 × 104 kg, which corresponds to a volume of 32.0 ± 3.6 m3 of lava being removed by the explosion. Data related to the texture and morphology of the ejected clasts were used to constrain a model of lava-snow interaction. The results suggest that the mechanism causing the explosions was the progressive build-up of pressure due to vapor accumulation under the lava flow, while no evidence was found for the occurrence of fuel-coolant interaction processes. Although these low-intensity explosions are not particularly frequent, the data set collected provides, for the first time, quantitative information about the processes involved and the associated hazard and suggests that mitigation measures should be established to prevent potentially dramatic accidents at worldwide volcanoes frequented by tourists and with fairly easy access, such as Etna.</jats:p>

<jats:p>The Samba Cu deposit is located in pre-Katangan Supergroup basement metavolcanic rocks of the Central African Copperbelt in Zambia. To better understand the formation of the Cu deposit, we use geochronologic, geochemical, and S isotopic data obtained for the deposit’s host rocks and sulfide minerals. Laser ablation−inductively coupled plasma−mass spectrometric (LA-ICP-MS) dating of zircon from host rocks, including quartz-sericite schist and biotite-sericite-chlorite schist, yields ages of 1967 ± 17 Ma and 1960 ± 20 Ma, which are interpreted as the crystallization ages of the original volcanic rocks. Re-Os analyses of molybdenite intergrown with chalcopyrite constrain sulfide deposition to 548.1 ± 7.5 Ma and 524.1 ± 7.3 Ma, which correspond to the timing of the Cu mineralization. LA-ICP-MS U-Pb analyses of metamorphic rutile from the biotite-sericite-chlorite schist yield a U-Pb age of 508.3 ± 6.6 Ma, which is interpreted to represent the age of regional metamorphism. The new data suggest that the Cu mineralization at Samba was synchronous with the Lufilian orogeny.</jats:p>

<jats:p>Composite Ni-Cu sulfide- and Fe-Ti oxide-bearing mafic-ultramafic intrusions have been discovered in convergent margin settings worldwide, but their origins remain enigmatic. Permian Kebu mafic-ultramafic intrusion in the Central Asian Orogenic Belt hosts both Ni-Cu sulfide and Fe-Ti oxide mineralization and formed in a post-collisional, Andean-style arc setting. The Ni-Cu sulfide- and Fe-Ti oxide-bearing rocks in the intrusion have similarly arc-like trace element patterns on the primitive mantle−normalized trace element diagram and negative εNd(t) values (−16 to −5), which indicate that the parental magma of the intrusion may have been derived from the enriched, metasomatized lithospheric mantle. The magma oxygen fugacity (fO2) was calculated to be FMQ+1.3∼FMQ+2.0, which can be attributed to the evolution of hydrated parental magma derived from the metasomatized lithospheric mantle. Also, the rocks all have negative δ13C values (−28.5‰ to −20‰), which indicates incorporation of external-derived crustal organic carbon into magmas. However, the Ni-Cu sulfide-bearing rocks contain more high-temperature carbon than the Fe-Ti oxide-bearing rocks. Modeling results based on alphaMELTS show that the magmas forming the Ni-Cu sulfide-bearing rocks may have assimilated more organic carbon than those that formed the Fe-Ti oxide-bearing rocks. Based on the estimated amounts of organic carbon in the two types of rocks, we deduced that initially oxidized magma pulses may have been reduced in variable degrees, leading to the formation of composite Ni-Cu sulfide and Fe-Ti oxide mineralization in one intrusive body. Mafic magmas in subduction-related, convergent margin settings are commonly highly oxidized due to metasomatism of the mantle wedge by slab-derived fluids/melts, and this may explain why the mafic-ultramafic intrusions associated with composite Ni-Cu sulfide and Fe-Ti oxide mineralization often occur in convergent margin settings.</jats:p>

<jats:p>Currently, the climatic implications associated with the Cenozoic tectonic history and growth mechanisms of the Tibetan Plateau lack consensus and remain controversial. This is due in part to chronological uncertainties and few paleoelevation records distributed in the central to northern Tibetan Plateau, which we address here with the development of a robust chronology (using magnetostratigraphy, biostratigraphy, detrital zircons, and regional radiochronologic dating) and a paleoelevation reconstruction for the Tuotuohe Basin (central-northern Tibet). We refined the age of the Tuotuohe Formation (37−33 Ma), Yaxicuo Formation (33−23.6 Ma), and Wudaoliang Formation (23.6−19.7 Ma). We estimated early Oligocene (ca. 29 Ma) paleotemperatures of the Tuotuohe Basin from 11 °C to 16.1 °C, which correspond to paleoelevations of 2.9 km (±0.4 km) when the relative humidity is 64% and 2.5 km (±0.5 km) when this value is 75%, using various methods including ostracod assemblages, gastropods, charophytes, branched glycerol dialkyl glycerol tetraether analysis, regional empirical formulas, and climate model simulation. Paleoelevation data and existing geological evidence in the vicinity indicate that late Eocene to late Oligocene uplift was associated with upper-crustal shortening. Since the middle Miocene, uplift has been associated with convective removal of lithospheric mantle and/or lower-crustal flow beneath the Hoh Xil Basin.</jats:p>