Interactive Material. Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

<p></p><p>Figures 2–5, 7–9, and 13 are interactive. For Figures 2–5, 9, and 13, use the radio buttons to toggle between different maps. For Figure 7, use the radio buttons to toggle between age range variations and uncertainties for different samples in each pluton. For Figure 8, use the radio buttons to toggle between zircon trace element compositions of different plutons.</p><p></p>

Unveiling ductile deformation during fast exhumation of a granitic pluton in a transfer zone

&lt;p&gt;Exhumation and cooling of upper crustal plutons is generally assumed to develop in the brittle domain, thus determining an abrupt passage from crystallization to faulting. To challenge this general statement, we have applied an integrated approach involving meso- and micro-structural studies, thermochronology, geochronology and rheological modeling. We have analyzed the Miocene syn-tectonic Porto Azzurro pluton on Elba (Tuscan archipelago &amp;#8211; Italy), emplaced in an extensional setting, and have realized that its fast exhumation is accompanied by localized ductile shear zones, developing along dykes and veins, later affected by brittle deformation. This is unequivocally highlighted by field studies and the analysis of microstructures with EBSD. In order to constrain the emplacement and exhumation rate of the Porto Azzurro pluton we performed U-Pb zircon dating and (U+Th)/He apatite thermochronology. It results in a magma emplacement age of 6.4 &amp;#177; 0.4 Ma and an exhumation rate of 3.4 to 3.9 mm/yr. By thermo-rheological modeling we were able to establish that localized ductile deformation occurred at two different time steps: within felsic dykes when the pluton first entered into the brittle field at 380 kyr, and along quartz-rich hydrothermal veins at c. 550 kyr after pluton emplacement. Hence, the major conclusion of our data is that ductile deformation can affect a granitic intrusion even when it is entered into the brittle domain in a fast exhuming extensional regime.&lt;/p&gt;

Petrogenesis of Mesoproterozoic Granites of the Swartoup Hills Region, Kakamas Domain, Namaqua Belt, South Africa

The Swartoup and Polisiehoek plutons in the Swartoup Hills (South Africa) formed during an episode of significant magma emplacement in the Mesoproterozoic Namaqua Sector of the Namaqua Metamorphic Province. They intruded into mid-crustal metasedimentary rocks of the metapelitic Koenap and mafic to carbonate-bearing Bysteek Formations during and shortly after the ∼1,200–1,220 Ma regional metamorphic peak that reached ultrahigh temperatures. Subsequent to pluton emplacement, the crust underwent regional high-temperature deformation during slow near-isobaric cooling. A further episode of pluton emplacement associated with fluid circulation truncated the first-order regional tectonic structures at ∼1,100 Ma. Based on their petrography, the Swartoup pluton is subdivided into leuco-granitoids with biotite as the sole mafic phase, pyroxene granitoids, and garnet-bearing granitoids, which may contain significant biotite. These subgroups display distinctive geochemical variations from one another, and from the Koenap Formation migmatites and the Polisiehoek granites, which are exposed nearby. Incompatible trace element distributions suggest that the Swartoup and Polisiehoek granitoids represent modified A-type granite magma, consistent with derivation from partial melting of quartzo-feldspathic crust. The magmas incorporated significant amounts of juvenile mantle-derived magma (εNd1200 of ∼−5, and LREE-depleted), but do not require older, early to late Paleoproterozoic crust. Particularly close to contacts to the calcic Bysteek Formation, localized contamination of the Swartoup granites by supracrustal carbonates is evident. A relatively pervasive alkali metasomatic effect is manifested strongly in the initial 87Sr/86Sr and LILE profiles of the Polisiehoek granites in particular, as well as in some of the Swartoup pyroxene granitoids, which could be either a symptom of CO2 metasomatism related to the Bysteek Formation carbonates, or to post-magmatic fluid metasomatism, perhaps linked to regional shearing. The comparison of our results with literature data suggests that similar sources, A-type granitic, Meso- to Paleoproterozoic crustal, and enriched mantle, have contributed, in locally differing proportions, to granites in most parts of the Namaqua Sector. Most likely, these plutons were generated during crustal and mantle melting in a long-lived hot continental back-arc environment.

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

<p></p><p>Figures 2–5, 7–9, and 13 are interactive. For Figures 2–5, 9, and 13, use the radio buttons to toggle between different maps. For Figure 7, use the radio buttons to toggle between age range variations and uncertainties for different samples in each pluton. For Figure 8, use the radio buttons to toggle between zircon trace element compositions of different plutons.</p><p></p>

Supplemental Material: Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

<div>Age and trace chemistry for Merrimac and other plutons, recalculation of previous Merrimac ages, and examples of Merrimac pluton textures. <br></div>

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

<p></p><p>Figures 2–5, 7–9, and 13 are interactive. For Figures 2–5, 9, and 13, use the radio buttons to toggle between different maps. For Figure 7, use the radio buttons to toggle between age range variations and uncertainties for different samples in each pluton. For Figure 8, use the radio buttons to toggle between zircon trace element compositions of different plutons.</p><p></p>

Supplemental Material: Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

<div>Age and trace chemistry for Merrimac and other plutons, recalculation of previous Merrimac ages, and examples of Merrimac pluton textures. <br></div>

Simultaneous Magmatic and Hydrothermal Regimes in Alta–Little Cottonwood Stocks, Utah, USA, Recorded Using Multiphase U-Pb Petrochronology

Magmatic and hydrothermal systems are intimately linked, significantly overlapping through time but persisting in different parts of a system. New preliminary U-Pb and trace element petrochronology from zircon and titanite demonstrate the protracted and episodic record of magmatic and hydrothermal processes in the Alta stock–Little Cottonwood stock plutonic and volcanic system. This system spans the upper ~11.5 km of the crust and includes a large composite pluton (e.g., Little Cottonwood stock), dike-like conduit (e.g., Alta stock), and surficial volcanic edifices (East Traverse and Park City volcanic units). A temperature–time path for the system was constructed using U-Pb and tetravalent cation thermometry to establish a record of >10 Myr of pluton emplacement, magma transport, volcanic eruption, and coeval hydrothermal circulation. Zircons from the Alta and Little Cottonwood stocks recorded a single population of apparent temperatures of ~625 ± 35 °C, while titanite apparent temperatures formed two distinct populations interpreted as magmatic (~725 ± 50 °C) and hydrothermal (~575 ± 50 °C). The spatial and temporal variations required episodic magma input, which overlapped in time with hydrothermal fluid flow in the structurally higher portions of the system. The hydrothermal system was itself episodic and migrated within the margin of the Alta stock and its aureole through time, and eventually focused at the contact of the Alta stock. First-order estimates of magma flux in this system suggest that the volcanic flux was 2–5× higher than the intrusive magma accumulation rate throughout its lifespan, consistent with intrusive volcanic systems around the world.

The cooling, deformation and exhumation history of the late Miocene syn-tectonic Porto Azzurro pluton in a regional transfer zone (Elba Island, Italy)

&lt;p&gt;In extensional tectonic settings, stretched terrains are often associated to lithosphere partial melting and widespread magmatism with plutons emplaced in the thinned crust. Emplacement of felsic magmas, at upper crustal levels, represents the final stage of the magma transfer from profound to shallow depth. In this framework, a mostly vertical permeability controls the magma uprising migration, as induced by dominant transcurrent crustal structures. Nevertheless, the interplay between extension and prolonged heat transfer favors uplift and progressive exhumation of the magmatic bodies, during their cooling.&lt;/p&gt;&lt;p&gt;In this presentation, we show an example of a felsic magmatic intrusion, the Porto Azzurro pluton (inner northern Apennines), emplaced in an extensional tectonic setting and mainly controlled by a regional transfer zone related to the opening of the Tyrrhenian Basin. This is exposed in the eastern Elba Island (Tuscan Archipelago). The hosting rocks of the Porto Azzurro pluton are mainly represented by micaschist, paragneiss and quartzite, affected by contact metamorphism and intense fluid circulation. We have analysed the structures that assisted the pluton emplacement and the ones that deformed the pluton itself during its cooling, from melt-present to brittle conditions, based on the integration among fieldwork, micro-structural, petrological and EBSD analyses. Furthermore, new U/Pb geochronological data on zircons and (U-Th)/He on apatite fission track refined the age of the pluton emplacement and its cooling, adding new data about the pluton history. Existing petrological analyses of the hosting rocks allowed us to better constrain the time-evolution of the thermal perturbation, permitting to frame the deformation and exhumation history of the Porto Azzurro monzogranite in the context of the Neogene extensional tectonics affecting the inner Northern Apennines.&lt;/p&gt;