The Appalachian Mountain range formed from multiple orogen-scale collisional events spanning from the Neoproterozoic to the Devonian. As the Iapetus Ocean began to close, early subduction zone magmatism created forearc lithosphere and volcanic arcs that were subsequently obducted onto Laurentia during the formation of Pangaea. These early magmatic products contain geochemical information that informs our understanding of subduction zone formation and evolution; however multiple collisional events, the emplacement of large (and potentially unrelated) magmatic intrusions, and extreme weathering at the southernmost extent of the Appalachians have made identifying these rocks and reconstructing the history of this portion of the margin notoriously difficult. Fortunately, recent studies of the modern Izu–Bonin–Mariana subduction system have shed new light on the geochemical evolution of subduction initiation and have resulted in the development of new criteria for classifying the first magmatic products of subduction–forearc basalts and boninites. These constraints allow for reassessment of subduction-related rocks throughout the Appalachian margin. Only one potential volcanic arc has been identified in the Southern Appalachians. This hypothesized arc, the Dadeville Complex of Alabama and Georgia, has geochemical signatures associated with subduction zone influence and is considered to have formed during subduction of Iapetan lithosphere underneath or adjacent to the Laurentian margin; however, the exact nature of the complex and its geologic history remains unclear. We apply the new understanding of subduction zone magmatic evolution derived from the Izu–Bonin–Mariana system to the Dadeville Complex to further elucidate its origin. Using whole rock and mineral major- and trace-element geochemistry coupled with detailed petrographic analyses, we have identified forearc basalts and boninites; therefore, we interpret these data to reflect formation of the Dadeville Complex during initial subduction in the Iapetus Ocean.
Abstract Metapelites from the inverted Barrovian sequence in the Sikkim Himalaya (northeast India) are shown to be largely continuous with respect to their bulk rock compositions, microstructures and pressure–temperature–time–deformation ( P – T – t – D ) histories. However, the upper garnet–lower staurolite zone demarcates a region of microstructurally anomalous post‐kinematic garnet populations contained within strongly segregated matrices. The different microstructures within samples from this region cannot be attributed to differences in their thermobarometric histories or bulk compositions, but are instead interpreted to represent an otherwise unexposed level of the Daling Group that is now exposed along a post‐metamorphic thrust splay. These heterogeneous samples contain several discrete garnet populations that progressively crystallized with increasing P – T . Garnet populations that experienced the most protracted growth now form complex polycrystals that exhibit crystallographically controlled and morphologically irregular interfaces adjacent to micaceous and quartzofeldspathic domains respectively. Electron backscatter diffraction indicates that these polycrystalline garnet structures contain numerous coalesced porphyroblasts that are structurally uncorrelated across their grain boundaries. However, a crystallographically preferred orientation at the polycrystal scale is interpreted to derive from epitaxial crystallization of early‐formed garnet porphyroblasts on precursor mica. Later‐nucleated porphyroblasts within polycrystals preferentially concentrated towards quartzofeldspathic domains, with the overall nucleation distribution likely controlled by a complex interplay between chemical heterogeneities, strain partitioning and epitaxial crystallization. The subsequent growth of these polycrystals was equally spatially heterogeneous; it was moderated by differences in the efficiency of grain boundary transfer between quartzofeldspathic and micaceous domains that precluded thin section‐scale chemical equilibration. In contrast to samples from Sikkim containing more typical porphyroblastic populations in continuous and disseminated matrices, heterogeneous availability of garnet‐forming components within this strongly layered matrix is shown to have resulted in grain‐scale variations in growth rates and the spatial juxtapositioning of interface‐controlled microstructures and locally equilibrated chemical compositions with those that were transport controlled.
<p>During subduction, devolatization reactions within the downgoing slab release significant volumes of fluid. Once released, the fate of such fluids remains unclear; they may either stagnate such that local rock systems remain undrained, or fluids may be mobilized over large length scales, draining the dehydrating rock volume. The fact that there is evidence from the metamorphic rock record to support both open- and closed-system fluid behavior demonstrates that permeability in deep crystalline metamorphic rock is both spatially and temporally heterogeneous. Prograde eclogitic veins greater than cm-scale are volumetrically scarce in the high pressure&#8211;low temperature (<em>HP&#8211;LT</em>) rock record, suggesting that either transient channelized flow is incredibly efficient and thus necessitates negligible grain boundary transfer and a low intact rock permeability, or that a large proportion of fluid migration to the subduction interface may be via more elusive grain boundary mechanisms.</p><p>Major element electron microprobe maps of <em>HP&#8211;LT</em> garnets from metabasic rocks of the Urals, Russia, As Sifa, Oman, and Syros, Greece, variably reveal short-wavelength and concentric oscillatory zoning in the outer rim region. Oscillatory zoning in most garnets is accompanied by homogeneous core-to-rim aluminum content. However, in samples from As Sifa and Syros, the onset of near-rim major element oscillatory zoning is concomitant with a rimwards step increase in Al content. Secondary ion mass spectrometry (SIMS) O-isotope analyses across rhythmic zoning in samples from each setting are used to assess the hypothesis that this sharp, stepwise change in garnet chemistry reflects a period of localized open system fluid-fluxing behavior, superimposed on a history of an otherwise stagnant fluid within an impermeable grain boundary network. In such a case, coupled oscillatory zoning in major and trace elements&#8212;as revealed by laser ablation&#8211;inductively coupled plasma&#173;&#8211;mass spectrometry (LA&#8211;ICP&#8211;MS) mapping&#8212;may point to pulsed <em>P&#8211;T</em> fluctuations, variable partitioning behavior, local kinetic effects associated with metamorphic reaction/dehydration, or changes in redox state serving as a driver for the development of this characteristic <em>HP&#8211;LT</em> geochemical garnet zoning.</p>
Abstract 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 ultramafic-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.
Subduction facilitates the transfer of volatiles from the Earth’s surface to its interior. However, the rock-scale processes that govern the efficiency of deep volatile transfer are not fully understood. Garnets from subduction zone rocks commonly have fine-scale, oscillatory elemental zoning that is typically considered to record external fluid ingress/transfer. Elemental and oxygen-isotope zoning in garnets from five exhumed subduction zone complexes show that in subduction zone rocks these records are not necessarily coupled; oxygen isotope evidence of ingress of buffering fluids, obvious only in rare cases, is decoupled from shorter length scale elemental and oxygen isotope zonings (which also show no coupling with each other). This finding suggests multiple mechanisms of internal chemical transfer operate at the grain and rock scale during subduction, and that rocks may commonly experience only limited interaction with external fluids. The results presented are consistent with a picture of volatile transfer in subduction that is spasmodic, highly localized, and variably efficient at evacuating fluids inherited from the surface then released by metamorphic dehydration.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)
