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Orogenic ophiolites are a hallmark of Phanerozoic plate tectonics, containing igneous lithologies that provide constraints on fundamental tectono-magmatic processes. The c. 1900Ma Pembine Ophiolite (Wisconsin, USA) is associated with the Penokean Orogen and represents a rare example of a proposed Paleoproterozoic ophiolite. The Penokean Orogen shares broad characteristics with Phanerozoic (<541 Ma) orogens, but the origin of the Pembine Ophiolite remains unclear, with the mafic volcanic rocks interpreted as representing either an intra-oceanic arc or continental back arc setting. To test these hypotheses, we present the results of petrography, bulk-rock geochemistry and mineral chemistry for a suite of 34 Pembine rocks, as well as U-Pb zircon geochronology for two samples. Based on trace elements established as immobile in the studied rocks, we demonstrate that mafic volcanism progressed (up-stratigraphic-section) from mid-ocean ridge-like to boninitic. The chemical evolution is identical to that observed in < 250 Ma ophiolites in the Himalayan–Alpine Orogen, which record forearc spreading during the nascent stages of subduction in the Tethys Ocean. We interpret the Pembine Ophiolite as forearc lithosphere formed during subduction initiation and obducted to the margin of the Superior Craton during the Penokean Orogeny. The processes responsible for forming (and preserving) this example of a Paleoproterozoic ophiolite may not have been dissimilar to those operating on the Phanerozoic Earth. 

Trace element concentrations and ratios in zircon provide important indicators of the petrological processes that operate in igneous and metamorphic systems. In granitoids, the compositions of zircon have been linked to the behaviour of garnet and plagioclase—pressure-sensitive minerals—in the source during partial melting. This has led to the proposal that Europium anomalies in detrital zircon are linked to the depth of crustal melting or magmatic differentiation and are a proxy for average crustal thickness. In addition to the mineral assemblage present during partial melting, Eu anomalies in zircon are also sensitive to redox conditions as well as magma evolution during extraction, ascent, and emplacement. Here we quantitatively model how rock type, mineral assemblages, redox changes, and reaction sequences influence Eu anomalies of zircon in equilibrium with silicate melt. Partial melting of metasedimentary rocks and metabasites yields felsic to intermediate melts with a large range of Eu anomalies, which do not correlate simply with pressure (i.e. depth) of melting. Europium anomalies of zircon associated with partial melting of metasedimentary rocks are most sensitive to temperature whereas Eu anomalies associated with metabasite melting are controlled by plagioclase proportion—a function of pressure, temperature, and rock composition—as well as changes in oxygen fugacity. Furthermore, magmatic crystallization of granitoids can increase or decrease Eu anomalies in zircon from those of the initial (anatectic) melt. Therefore, Eu anomalies in zircon should not be used as a proxy for the crustal thickness or depth of melting but can be used to track the complex processes of metamorphism, partial melting, and magmatic differentiation in modern and ancient systems. Secular changes of Eu/Eu* from the zircon archive may reflect a change in thermal gradients of crustal melting or an increase in the reworking of sedimentary rocks over time. 

Mammals rose to prominence in terrestrial ecosystems after the Cretaceous–Paleogene mass extinction, but the mammalian lineages characteristic of Paleogene faunas began their evolutionary and ecological diversification in the Late Cretaceous, stimulated by the rise of angiosperms (flowering plants) according to the preeminent hy- pothesis. The Cretaceous rise of mammals is part of a larger expansion in biodiversity on land that has been termed the Cretaceous (or Angiosperm) Terrestrial Revolution, but the mechanisms underlying its initiation remain opaque. Here, we review data from the fossil and rock records of western North America—due to its relatively continuous fossil record and complete chronology of mountain-building events—to explore the role that tectonism might have played in catalyzing the rise of modern-aspect terrestrial biodiversity, especially that of mammals and angiosperms. We highlight that accelerated increases in mammal and angiosperm species richness in the Late Cretaceous, ca. 100–75 Ma, track the acceleration of tectonic processes that formed the North American Cordillera and occurred during the ‘middle-Cretaceous greenhouse’ climate. This rapid increase in both mammal and angiosperm diversity also occurred during the zenith of Western Interior Seaway trans- gression, a period when the availability of lowland habitats was at its minimum, and oscillatory transgression- regression cycles would have frequently forced upland range shifts among lowland populations. These changes to both landscapes and climates have all been linked to an abrupt, global tectonic-plate ‘reorganization’ that occurred ca. 100 Ma. That mammals and angiosperms both increased in species richness during this interval does not appear to be a taphonomic artifact—some of the largest spikes in diversity occur when the available mammal-bearing fossil localities are sparse. Noting that mountainous regions are engines for generating biodi- versity, especially in warm climates, we propose that the Cretaceous/Angiosperm Terrestrial Revolution was ultimately catalyzed by accelerated tectonism and enhanced via cascading changes to landscapes and climate. In the fossil record of individual basins across western North America, we predict that (1) increases in mammalian diversity through the Late Cretaceous should be positively correlated with rates of tectonic uplift, which we infer to be a proxy for topographic relief, and are attended by increased climate heterogeneity, (2) the diversity of mountain-proximal mammalian assemblages should exceed that of coeval mountain-distal assemblages, espe- cially in the latest Cretaceous, and (3) endemism should increase from the latest Cretaceous to early Paleogene as Laramide mountain belts fragmented the Western Interior. Empirical tests of these predictions will require increased fossil collecting in under-sampled regions and time intervals, description and systematic study of existing collections, and basin-scale integration of geological and paleontological data. Testing these predictions will further our understanding of the coevolutionary processes linking tectonics, climate, and life throughout Earth history. 

Abstract Eclogite thermobarometry is crucial for constraining the depths and temperatures to which oceanic and continental crust subduct. However, obtaining the pressure and temperature (P–T) conditions of eclogites is complex as they commonly display high‐variance mineral assemblages, and the mineral compositions only vary slightly withP–T. In this contribution, we present a comparison between two independent and commonly used thermobarometric approaches for eclogites: conventional thermobarometry and forward phase‐equilibrium modelling. We assess how consistent the thermobarometric calculations are using the garnet–clinopyroxene–phengite barometer and garnet–clinopyroxene thermometer with predictions from forward modelling (i.e. comparing the relative differences between approaches). Our results show that the overall mismatch in methods is typically ±0.2–0.3 GPa and ±29–42°C although differences as large as 80°C and 0.7 GPa are possible for a few narrow ranges ofP–Tconditions in the forward models. Such mismatch is interpreted as the relative differences among methods, and not as absolute uncertainties or accuracies for either method. For most of the investigatedP–Tconditions, the relatively minor differences between methods means that the choice in thermobarometric method itself is less important for geological interpretation than careful sample characterization and petrographic interpretation for derivingP–Tfrom eclogites. Although thermobarometry is known to be sensitive to the assumedX_Fe³⁺of a rock (or mineral), therelativedifferences between methods are not particularly sensitive to the choice of bulk‐rockX_Fe³⁺, except at high temperatures (>650°C, amphibole absent) and for very large differences in assumedX_Fe³⁺(0–0.5). We find that the most important difference between approaches is the activity–composition (a–x) relations, as opposed to the end‐member thermodynamic data or other aspects of experimental calibration. When equivalenta–xrelations are used in the conventional barometer,Pcalculations are nearly identical to phase‐equilibrium models (ΔP < 0.1). To further assess the implications of these results for real rocks, we also evaluate common mathematical optimizations of reaction constants used for obtaining the maximumP–Twith conventional thermobarometric approaches (e.g. using the highestaGrs² × aPrp in garnet and Si content in phengite, and the lowestaDi in clinopyroxene). These approaches should be used with caution, because they may not represent the compositions of equilibrium mineral assemblages at eclogite facies conditions and therefore systematically biasP–Tcalculations. Assuming method accuracy, geological meaningfulP_maxat a typical eclogite facies temperature of ~660°C will be obtained by using the greatestaDi,aCel, andaPrp and lowestaGrs andaMs; garnet and clinopyroxene with the lowest Fe²⁺/Mg ratios may yield geological meaningfulT_maxat a typical eclogite facies pressure of 2.5 GPa. 

To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ~160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ~180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ~60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process. 

Suprasubduction zone (SSZ) ophiolites of the northern Appalachians (eastern North America) have provided key constraints on the fundamental tectonic processes responsible for the evolution of the Appalachian orogen. The central and southern Appalachians, which extend from southern New York to Alabama (USA), also contain numerous ultra- mafic-mafic bodies that have been interpreted as ophiolite fragments; however, this interpretation is a matter of debate, with the origin(s) of such occurrences also attributed to layered intrusions. These disparate proposed origins, alongside the range of possible magmatic affinities, have varied potential implications for the magmatic and tectonic evolution of the central and southern Appalachian orogen and its relationship with the northern Appalachian orogen. We present the results of field observations, petrography, bulk-rock geochemistry, and spinel mineral chemistry for ultramafic portions of the Baltimore Mafic Complex, which refers to a series of ultramafic-mafic bodies that are discontinuously exposed in Maryland and southern Pennsylvania (USA). Our data indicate that the Baltimore Mafic Complex comprises SSZ ophiolite fragments. The Soldiers Delight Ultramafite displays geochemical characteristics—including highly depleted bulk-rock trace element patterns and high Cr# of spinel—characteristic of subduction-related mantle peridotites and serpentinites. The Hollofield Ultramafite likely represents the “layered ultramafics” that form the Moho. Interpretation of the Baltimore Mafic Complex as an Iapetus Ocean–derived SSZ ophiolite in the central Appalachian orogen raises the possibility that a broadly coeval suite of ophiolites is preserved along thousands of kilometers of orogenic strike. 

Abstract We present a comprehensive petrological and geochronological study of a single granulite sample from the lithosphere‐scale Beraketa shear zone in southern Madagascar to constrain the orogenic history of Gondwana assembly in this region. The studied sample provides a panoply of data constraining the prograde, retrograde, and late metasomatic history of the region via the application of Ti‐in‐quartz, Ti‐in‐zircon, Zr‐in‐rutile, and Al‐in‐orthopyroxene thermobarometry; phase‐equilibrium modelling; U–Pb monazite, zircon, and rutile petrochronology; and trace element diffusion chronometry in rutile. Our results reveal five stages of metamorphism along a narrow clockwiseP–Tpath that may have begun as early as 620–600 Ma and certainly by 580–560 Ma, based on the oldest concordant zircon dates. The rock was heated to >725°C at less than 7.5 kbar (Stage 1) before burial to ~8 kbar (Stage 2). Byc. 540 Ma, the rock had heated to ~970°C at ~9 kbar, and lost approximately 12% melt (Stage 3), before decompressing and cooling to the solidus at ~860°C and 6.5 kbar within 10 Ma (Stage 4). The vast majority of monazite and zircon dates record Stage 4 cooling and exhumation. Monazite and zircon rim dates as young asc. 510 Ma record subsolidus cooling (Stage 5) and associated symplectite formation around garnet. U–Pb rutile dates record partial resetting atc. 460 Ma; Zr‐ and Nb‐in‐rutile diffusion chronometry link these dates to a metasomatic event that lasted <1 Ma at ~600°C. In addition to chronicling a near‐complete cycle of metamorphism in southern Madagascar, this study constrains the rates of heating and cooling. We estimate that heating (7–14°C/Ma) outpaced reasonable radiogenic heating rates with modest mantle heat conduction. Therefore, we conclude that elevated mantle heat conduction or injection of mantle‐derived magmas likely contributed to regional ultrahigh‐temperature metamorphism (UHTM). Exhumation and cooling from peak metamorphic conditions to the solidus occurred at rates greater than 0.45 km/Ma and 14°C/Ma.