Jiří Žák

Characteristics of internal contacts in the Tuolumne Batholith, central Sierra Nevada, California (USA): Implications for episodic emplacement and physical processes in a continental arc magma chamber

Internal contacts along the eastern margin of the Tuolumne Batholith, central Sierra Nevada, California, and structures preserved along these contacts, are highly variable; the contacts range from relatively sharp, to gradational boundaries, to sheeted zones, to very complex boundaries formed by multiple processes. Fractional crystallization, kilometer-scale mixing within broad transition zones, voluminous magmatic stoping along sharp contacts, and downward return flow (and/or margin collapse) of older magma units were important large-scale processes along these contacts during chamber construction. In contrast, sheeting, extensional cracking and diking represent only second-order, small-scale complexities. Formation of the most complex zones along internal contacts resulted from the interaction of the sequential emplacement of different units with the irregular geometry of these contacts, which often resulted from removal of earlier phases by stoping. Our results indicate that multiple processes are likely during emplacement of large magma bodies within one another and that it is unlikely that evidence for all internal processes during batholith construction are preserved. We also argue that fairly large magma chambers existed in this batholith and thus that large accumulations of magma may exist in upper crustal chambers for significant periods of time.

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.

Dating the onset of Variscan crustal exhumation in the core of the Bohemian Massif: new U–Pb single zircon ages from the high-K calc-alkaline granodiorites of the Blatná suite, Central Bohemian Plutonic Complex

Abstract: The Variscan Central Bohemian Plutonic Complex crops out between the upper-crustal Teplá–Barrandian and the high-grade Moldanubian units (Bohemian Massif). Much of the complex is made up of calc-alkaline plutonic rocks: (1) the geochemically more primitive, Na-rich 354 ± 4 Ma Sázava suite, which was emplaced syntectonically during regional shortening; (2) the younger, more evolved, potassic Blatná suite, which records both the shortening along its NW contact and the onset of normal shearing related to exhumation of the Moldanubian Unit to the SE. New ion microprobe U–Pb zircon ages for the high-K calc-alkaline Blatná suite pinpoint this major switch in the tectonic regime. Both Blatná and Kozárovice granodiorites (346 ± 2 Ma and 347 ± 2 Ma (2σ)) were generated by melting of heterogeneous crust and were emplaced contemporaneously to form the Blatná composite pluton. From age spectra preserved in inherited zircons and whole-rock Sr–Nd isotope signatures, the likely crustal sources for the magmas were immature greywackes rich in Neoproterozoic (615 ± 10 Ma, Kozárovice) or Late Cambrian–Early Ordovician (492 ± 4 Ma, Blatná) volcanogenic detritus. An additional important petrogenetic process was variable mixing with enriched mantle-derived monzonitic magmas, which may also have supplied the extra heat for crustal anatexis. Supplementary material: Analytical techniques, selected whole-rock major- and trace-element geochemical analyses and U–Th–Pb data are available at http://www.geolsoc.org.uk/SUP18391.

A plate-kinematic model for the assembly of the Bohemian Massif constrained by structural relationships around granitoid plutons

Abstract This paper summarizes the current knowledge on the nature, kinematics and timing of movement along major tectonic boundaries in the Bohemian Massif and demonstrates how the Variscan plutonism and deformation evolved in space and time. Four main episodes are recognized: (1) Late Devonian–early Carboniferous subduction and continental underthrusting of the Saxothuringian Unit beneath the Teplá–Barrandian Unit resulted in the orogen-perpendicular shortening and growth of an inboard magmatic arc during c. 354–346 Ma; (2) the subduction-driven shortening was replaced by collapse of the Teplá–Barrandian upper crust, exhumation of the high-grade (Moldanubian) core of the orogen at c. 346–337 Ma and by dextral strike-slip along orogen-perpendicular NW–SE shear zones; (3) following closure of a Rhenohercynian Ocean basin, the Brunia microplate was underthrust beneath the eastern flank of the Saxothuringian/Teplá–Barrandian/Moldanubian ‘assemblage’; this process commenced at c. 346 Ma in the NE and ceased at c. 335 Ma in the SW; and (4) late readjustments within the amalgamated Bohemian Massif included crustal exhumation and mainly S-type granite plutonism along the edge of the Brunia indentor at c. 330–327 Ma, and peripheral tectonothermal activity driven by strike-slip faulting and possibly mantle delamination around the consolidated Bohemian Massifs interior until late Carboniferous–earliest Permian times.

Multiple magmatic fabrics in plutons: an overlooked tool for exploring interactions between magmatic processes and regional deformation?

This paper elaborates on the concept of multiple magmatic fabrics in plutons. After a general overview of various types of multiple fabrics that may develop in magmatic rocks, two case examples of porphyritic granite and melasyenite plutons in the Bohemian Massif are examined. In the Jizera granite, complex variations in K-feldspar phenocryst shape-fabric revealed by image analysis of a 200 m long section of an underground tunnel are in contrast with homogeneously oriented magnetic (AMS) fabric carried by coaxial contributions of biotite, magnetite and maghemite. In the Knižeci Stolec melasyenite pluton, emplacement-related margin-parallel feldspar foliation was overprinted by flat-lying foliation; the latter is interpreted to record regional tectonic strain. At the two stations examined in detail, the crystallographic-preferred orientation of biotite and amphibole in the inter-phenocryst matrix (measured using electron back-scatter diffraction – EBSD) differed from both feldspar fabric and also from the AMS principal directions. Multiple magmatic fabrics in these two plutons are interpreted in terms of fabric superposition, where late weak strain is superposed onto a high-strength phenocryst framework, but is accommodated preferentially by small mineral grains (biotite, magnetite) in the melt-bearing matrix. This mechanism may explain the discrepancy between mesoscopic feldspar fabric and AMS. We conclude that multiple magmatic fabrics in plutons may thus result from accumulated strain caused by different processes during final crystallization and, as such, may serve as a sensitive indicator of the evolving interactions between magmatic and tectonic processes in the Earth’s crust.

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

Rapid extensional unroofing of a granite–migmatite dome with relics of high-pressure rocks, the Podolsko complex, Bohemian Massif

The Podolsko complex, Bohemian Massif, is a high-grade dome that is exposed along the suprastructure–infrastructure boundary of the Variscan orogen and records snapshots of its protracted evolution. The dome is cored by leucocratic migmatites and anatectic granites that enclose relics of high- to ultrahigh-pressure rocks and is mantled by biotite migmatites and paragneisses whose degree of anatexis decreases outwards. Our new U–Pb zircon ages indicate that the leucocratic migmatites were derived from Early Ordovician ( c. 480 Ma) felsic igneous crust; the same age is inferred for melting the proto-source of the metapelitic migmatites. The relics of high- to ultrahigh-pressure rocks suggest that at least some portions of the complex witnessed an early Variscan subduction to mantle depths, followed by high-temperature anatexis and syntectonic growth of the Podolsko dome in the middle crust at c. 340–339 Ma. Subsequently, the dome exhumation was accommodated by crustal-scale extensional detachments. Similar c. 340 Ma ages have also been reported from other segments of the Variscan belt, yet the significance of this tectonothermal event remains uncertain. Here we conclude that the 340 Ma age post-dates the high-pressure metamorphism; the high temperatures required to cause the observed isotopic resetting and new growth of zircon were probably caused by heat input from the underlying mantle and, finally, the extensional unroofing of the complex requires a minimum throw of about 8–10 km. We use this as an argument for significant early Carboniferous palaeotopography in the interior of the Variscan orogen.

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).