Valbone Memeti

Magma addition and flux calculations of incrementally constructed magma chambers in continental margin arcs: Combined field, geochronologic, and thermal modeling studies

Incrementally constructed magma systems have been recognized from studies of the resulting plutons for more than three decades. However, magma addition rates, fluxes, growth durations, sizes of increments, and sizes and durations of the resulting magma chambers have been difficult to ascertain, emphasizing the need for a better understanding of how magmatic systems evolve. Our results from studies of plutons and arc sections in the North American Cordillera indicate that a large range exists in all of these values. Although arc sections and individual plutons clearly have dramatic temporal changes in volumetric magma additions, true volumetric flux calculations are particularly difficult to determine. Thus, although subduction beneath arcs may have active durations of hundreds of millions of years, volumetrically most magmatism is emplaced during magmatic flare-ups of ∼10–30 m.y. duration. Individual plutons and batholiths in these arcs can grow in

Magmatic lobes as "snapshots" of magma chamber growth and evolution in large, composite batholiths: An example from the Tuolumne intrusion, Sierra Nevada, California

Precise chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) U-Pb zircon ages in combination with detailed field mapping, 40 Ar/ 39 Ar thermochronology, and finite difference thermal modeling in the magmatic lobes of the Tuolumne batholith characterize these 10–60 km 2 bodies as shorter-lived, simpler magmatic systems that represent increments of batholith growth. Lobes provide shorter-term records of internal and external processes that are potentially obliterated in the main body of long-lived, composite batholiths. Zircon ages complemented by thermal modeling indicate that lobe-sized magma chambers were present between ∼0.2 and 1 m.y., representing only a small fraction of the total duration of melt presence in the main body. During these shorter intervals, a concentric pattern of normal compositional zoning formed during inward crystallization and widespread zircon recycling in the lobes. Lobes largely evolved as individual magma bodies that did not interact significantly with the main, more complex magma chamber(s). Antecrystic zircons and the range of autocrysts, used to track the extent of interconnected melt, record only a limited range of ages and have contrasting zircon populations to those found in the same units in the main batholith. We consider lobes to either be single batches formed during continuous magma flow or multiple, quickly coalescing pulses that in either case formed separate magma chambers that failed to amalgamate with other compositionally distinct pulses such as those occurring in the central batholith. Zircon age comparisons between all four lobes and the main body imply that growth of the Tuolumne intrusion was not stationary, but that the locus of magmatism shifted both inward and northwestward.

Four magmatic fabrics in the Tuolumne batholith, central Sierra Nevada, California (USA): Implications for interpreting fabric patterns in plutons and evolution of magma chambers in the upper crust

For the first time, four distinct magmatic fabrics are documented in a composite plutonic body, the Tuolumne batholith, central Sierra Nevada, California, USA. One type of fabric was formed by strain caused by highly localized magma flow (type 1), whereas the other three chamber-wide fabrics recorded strain increments during boundary processes along batholith margins (type 2) superimposed by increments of heterogeneous regional tectonic strain (types 3 and 4). Our present work indicates that, in contrast to studies that consider magmatic fabrics to be simple structures formed by a single process, magmatic lineations and foliations in plutons may reflect accumulated finite strain and form as composite structures recording multiple strain increments in relatively static, actively deforming and rheologically complex crystal mushes at lower melt percentages. We demonstrate that multiple magmatic fabrics in a single batholith may record remarkably different processes and thus preserve a temporal record of strain in the batholith from strain during internal chamber processes to postem-placement regional tectonic strain. However, it may be commonly very problematic to infer the exact nature of flow or even fabric-forming process from the preserved rock record. Our study also exemplifies how examination of magmatic fabric patterns in plutons, complemented with geochronology, may provide crucial constraints on the interplay among successive magma emplacement, fabric preservation, temporal evolution of strain fields in a crystallizing magma chamber, and the development of crystal-mush zones.

Tracking paleodeformation fields in the Mesozoic central Sierra Nevada arc: Implications for intra-arc cyclic deformation and arc tempos

In this study, structures in plutons and host rocks are coupled with geochronology to track paleodeformation fields from the late Paleozoic to Late Cretaceous in the central Sierra Nevada. Regional NW-striking host-rock foliation, NE- or SW-vergent thrust faults, and associated folds developed from the early Mesozoic to Early Cretaceous. Dextral transpressional shear zones developed in the Late Cretaceous. Strikes of steep-dipping magmatic foliations in Mesozoic plutons temporally vary from approximately NW (Triassic–Jurassic) to WNW (Late Cretaceous), displaying a progressive counterclockwise rotation. Joint interpretation based on combining host-rock and magmatic structures suggests that intra-arc paleodeformation fields were dominated by coaxial and arc-perpendicular contraction from the early Mesozoic to Early Cretaceous, becoming increasingly dextral transpressive in the Late Cretaceous. The switch from contraction to transpression was likely caused by oblique convergence between the Farallon and North American plates. Based on observations in the study area and other host-rock pendants in the central Sierra Nevada, we propose that the intensity of intra-arc deformation is cyclic. To some extent, it mimics the episodic pattern of arc magmatism: Stronger deformation coincides with magmatic flare-ups. Magmatism promotes intra-arc deformation, which in turn causes crustal thickening during transfer of materials downward to the magma source regions, potentially fertilizing source regions with supracrustal materials and resulting in increased magma generation. Thus, models addressing continental arc tempos should include intra-arc processes. Evolution of continental arcs may be influenced by linked cyclic processes within the arcs accompanied by noncyclic processes driven by events external to the arcs.

Is stoping a volumetrically significant pluton emplacement process?: Discussion

[Glazner and Bartley (2006)][1] suggest that stoping is an insignificant to potentially nonexistent process in the emplacement and evolution of magmatic systems. We strongly disagree with this conclusion and present here a number of alternative perspectives, which we group into three categories:

Repeated, multiscale, magmatic erosion and recycling in an upper-crustal pluton: Implications for magma chamber dynamics and magma volume estimates

Abstract The Tuolumne Intrusive Complex, an upper-crustal (7–11 km emplacement depths), incrementally constructed (95–85 Ma growth history) plutonic complex (~1100 km2), preserves evidence from several data sets indicating the repeated, multiscale, magmatic erosion of older units occurred and that some eroded material was recycled into younger magma batches. These include: (1) map patterns of internal contacts (hundreds of kilometers) that show local hybrid units, truncations, and evidence of removal of older units by younger; (2) the presence of widespread xenolith and cognate inclusions (thousands), including “composite” inclusions; (3) the presence of widespread enclaves (millions), including “composite” enclaves, plus local enclave swarms that include xenoliths and cognate inclusions; (4) the presence of widespread schlieren-bound magmatic structures (>9000) showing evidence of local (meter-scale) truncations and erosion; (5) antecrystic zircons (billions) and other antecrystic minerals from older units now residing in younger units; (6) whole-rock geochemistry including major element, REE, and isotopic data; and (7) single mineral petrographic and geochemical studies indicating mixing of distinct populations of the same mineral. Synthesis of the above suggests that some erosion and mixing occurred at greater crustal depths, but that thousands of “erosion events” at the emplacement site resulted in removal of ~35–55% of the original plutonic material from the presently exposed surface with some (~25%?) being recycled into younger magmas and the remainder was either erupted or displaced downward. The driving mechanisms for mixing/recycling are varied but likely include buoyancy driven intrusion of younger batches into older crystal mushes, collapse and avalanching along growing and over-steepened solidification fronts within active magma chambers (1 to >500 km2 in size), and local convection in magma chambers driven by internal gradients (e.g., buoyancy, temperature, and composition).

Mineral fabrics in high-level intrusions recording crustal strain and volcano–tectonic interactions: the Shellenbarger pluton, Sierra Nevada, California

The Minarets caldera is a volcano–plutonic complex in the Sierra Nevada, California, that exemplifies complex interactions between volcanism and tectonic deformation in continental-margin arcs. Caldera evolution commenced with emplacement of pre-collapse rhyolitic ash-flow tuff, followed by collapse and deposition of volcanic breccia and rhyodacitic ash-flow tuff. Subsequently, the volcanic rocks were deformed along the regional Bench Canyon shear zone. The caldera centre was then intruded by the resurgent c. 100 Ma steep-sided Shellenbarger granite pluton, which steepened the shear zone foliation. The pluton was overprinted by syn- to post-magmatic ∼NNE–SSW horizontal shortening; the same shortening was documented in several other Late Cretaceous syntectonic plutons in the Sierra Nevada and interpreted to record dextral transpression during convergence of the Farallon and North American plates. To explain the unusual tectonic fabric in the shallow-level Shellenbarger pluton, we develop a general model for strain partitioning in syntectonic magma bodies emplaced at various crustal levels. We propose that shallow intrusions, isolated within stiff crust, may tend to accommodate minor pure shear strain whereas simple shear dominates along weak faults and shear zones. By contrast, a rheological reversal is crossed deeper in the crust and magma bodies become the weakest, simple shear-dominated parts of the system. Supplementary material: Analytical methods and anisotropy of magnetic susceptibility and U–Th–Pb isotopic data are available at https://doi.org/10.6084/m9.figshare.c.3582749

Deciphering magmatic processes in calc-alkaline plutons using trace element zoning in hornblende

Abstract Hornblende in the Kuna Crest lobe (KCL) of the Tuolumne Intrusive Complex (TIC) and the upper zone of the Wooley Creek batholith (WCB) precipitated over a temperature range of ~835 to 700 °C, and thus has the potential to record magmatic processes. We measured trace element concentrations in hornblende from the WCB, from the KCL of the TIC, and from one sample from an adjacent interior unit of the TIC to compare and contrast magmatic processes in these two mid-crustal intrusions. In both systems the magmatic amphibole is magnesiohornblende in which Ti, Zr, Hf, Nb, Sr, Ba, and rare earth elements (REE) typically decrease from crystal interiors to rims, an indication of compatible behavior of these elements, and the size of the negative Eu anomaly decreases. In the Kuna Crest lobe, hornblende from individual mapped units differs in trace element abundances and zoning trends. Some samples contain at least two distinct hornblende populations, which is particularly evident in the shapes of REE patterns. In contrast, compositions of hornblende from all structural levels of the upper WCB and related dacitic roof-zone dikes form a single broad array and the REE patterns are essentially indistinguishable, regardless of rock type, from quartz diorite to granite. In the WCB, Zr/Hf ratios in hornblende are consistent with crystallization from a melt with chondritic Zr/Hf values. In contrast, most hornblende in the KCL has Zr/Hf values lower than expected from crystallization from a melt with chondritic values, suggesting that zircon fractionation occurred before and during crystallization of the hornblende. Simple fractional crystallization models indicate that REE, high field strength elements, Sr, and Ba were compatible in KCL and WCB magmas as hornblende grew; these trends require removal of hornblende + plagioclase + zircon ± ilmenite ± biotite. The uniform variations of trace element concentrations and patterns in the upper WCB and roof-zone dikes indicates crystallization from a large magma body that was compositionally uniform; probably stirred by convection caused by influx of mafic magmas at the base of the zone (Coint et al. 2013a, 2013b; cf. Burgisser and Bergantz 2011). In contrast, in the KCL, each analyzed sample displays distinct hornblende compositions and zoning patterns, some of which are bimodal. These features indicate that each analyzed sample represents a distinct magma and that individual magmas were variably modified by fractionation and mixing. Hornblende trace element contents and zoning patterns prove to be powerful tools for identification of magma batches and for assessing magmatic processes, and thereby relating plutonic rocks to hypabyssal and volcanic equivalents.

Formation and transfer of stoped blocks into magma chambers: The high-temperature interplay between focused porous flow, cracking, channel flow, host-rock anisotropy, and regional deformation

Magmatic stoping, i.e., the formation, transfer into, and movement through magma of older plutonic and metamorphic host-rock xenoliths, was widespread in the Mesozoic Sierra Nevada batholith (California, United States). However, the prevailing view that stoped blocks form by rapid thermal shattering and collapse into chambers may not be the dominant process of block formation and displacement into chambers in the Sierra Nevada. In detailed studies in and around the Tuolumne Batholith and Jackass Lakes pluton, we found evidence for the following history of block formation in slightly older, fairly isotropic plutonic host rocks: (1) low stress sites developed, leading to planar zones of increased porosity; (2) focused porous flow of first felsic melts followed by intermediate melts led to growth of magma fingers, which in turn led to increased porosity and loss of host-rock cohesion; and (3) connection of magmatic fingers resulted in the formation of dike-like channels in which flow facilitated removal of all host-rock material in these planar zones. Once formed, blocks were initially displaced by repeated magma injections along these channels, often resulting in unidirectional growth in these zones creating local magmatic sheeted complexes along block margins. Free block rotation occurred when sufficient nonlayered magma surrounded the host block; in some cases, segments of former sheeted zones remain attached to rotated blocks. In anisotropic metamorphic host rocks, focused porous flow may have locally played a role, but the dominant processes during initial block formation were cracking, parallel and at high angles to anisotropy, and intrusion of magma by channel flow. Subsequent initial block displacement and eventual rotation are identical to those in the nearly isotropic host rock. The driving forces for the development of low-stress sites, cracking, dilation, and magma flow remain uncertain, but likely reflect the interplay between regional stress, magma buoyancy stresses, thermal gradients, and host-rock properties, and not simply rapid heating and thermal expansion cracking. Thus a number of processes may drive block formation, some of which are rapid (thermal shattering, roof collapse) whereas others occur over longer durations (incremental magma pulsing and formation of sheeted complexes, regional deformation).

Nested Incremental Growth of Zoned Upper Crustal Plutons in the Southern Uplands Terrane, UK: Fractionating, Mixing, and Contaminated Magma Fingers

&NA; The type and magnitude of the physical and chemical processes controlling magma ascent, storage and solidification in zoned plutons remain controversial, even though such plutons are widespread in arcs. Linking both plutonic and host rock characteristics is key towards determining quantitative models for the growth and evolution of these plutons. We report a synthesis of an integrated study of the Loch Doon pluton, an archetypal zoned pluton in the Southern Uplands Terrane of Scotland, including linked plutonic and host rock field mapping, structural analysis, LA‐ICP‐MS U‐Pb zircon geochronology, whole‐rock and isotope geochemistry and magnetic susceptibility analyses. Our results reveal that trapping of rising magma and construction of the pluton in the upper crust at ˜397 Ma was dominated by vertical host rock displacement: likely a combination of downward host rock flow, stoping and floor subsidence, and to a lesser extent upward doming and faulting. Mixing of depleted mantle‐derived and crustal melts, and fractionation and contamination of these at deeper crustal levels, was followed by continued in situ fractionation and mixing during crystallization that was extended by continual chamber rejuvenation and thermal buffering by recharge from magmas. Analysis of the plutons ubiquitous magmatic foliations and their relationship to internal magmatic and external host rock structures suggests that later, typically more evolved magma batches ‘nested’ within the rheologically weakest portions of earlier batches and made space by largely displacing these earlier intrusive batches rather than the external host rock. The active parts of the nesting system decreased with time, consistent with a freezing magma feeder system. This model is applicable to many other zoned late Caledonian Southern Uplands Terrane plutons of the United Kingdom. These Southern Uplands Terrane plutons appear to be magmatic ‘fingers’ rising from larger, deeper intrusive complexes that resided at depth. Our new data also allow us to present further constraints on the geodynamic model for this part of the Caledonian orogenic belt. Our interpretation of antecrystic and autocrystic zircon ages in Southern Uplands Terrane plutons indicates prolonged magmatism between ˜430–382 Ma and analysis of whole‐rock Sr/Y ratios suggests crustal thicknesses during magma generation may have varied in space and, or, time between ˜30–45 km. Our data best fit a tectonic model in which magmatism was initiated at ˜430 Ma in response to break‐off of subducted Iapetan lithosphere and was extended by additional melt/heat input during regional transtension at ˜415–400 Ma. Magmas evolved by complex open system fractionation, mixing, and contamination in a mid‐lower crustal batholith before being emplaced as compositionally zoned nested plutons in the upper crust between ˜414–382 Ma.