New age constraints on magmatism and metamorphism in the Morin terrane (Grenville Province, Quebec)

The Morin terrane is an allochthonous crustal block in the southwestern Grenville Province with a relatively poorly-constrained metamorphic history. In this part of the Grenville Province, some terranes were part of the ductile middle crust during the 1.09–1.02 Ga collision of Laurentia with the Amazon craton (the Ottawan phase of the Grenvillian orogeny), while other terranes were part of the orogen’s superstructure. New U-Pb geochronology suggests that the Morin terrane experienced granulite-facies metamorphism during the accretionary Shawinigan orogeny (1.19–1.14 Ga) and again during the Ottawan. Seven zircon samples from the 1.15 Ga Morin anorthosite suite were dated to confirm earlier age determinations, and Ottawan metamorphic rims (1.08–1.07 Ga) were observed in two samples. U-Pb dating of titanite in nine marble samples surrounding the Morin anorthosite suite yielded mixed ages spanning between the Shawinigan and Ottawan metamorphisms (n=7), and predominantly Ottawan ages (n=2). Our results show that Ottawan zircon growth and resetting of titanite ages is spatially heterogeneous in the Morin terrane. Ages with a predominantly Ottawan signature are recognized in the Morin shear zone, which deforms the eastern lobe of the anorthosite, in an overprinted skarn zone on the western side of the massif, and in the Labelle shear zone that marks its western boundary. In the rest of the Morin terrane titanite with Shawinigan ages appear to have been only partially reset during the Ottawan. Further work is needed to better understand the relationship between the character of Ottawan metamorphism and resetting in different parts of the Morin terrane.

Supplemental Material: Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

<div>Text S1: Supplemental text. Figure S1: Cathodoluminescence images for all analyzed zircon grains. Figure S2: REE spider plots for zircon. Figure S3: Tukey honestly significant difference (HSD) for the timing of anatexis. Table S1: Cathodoluminescence images for all analyzed zircon grains. Table S2. Grossular content of garnet used to calculate the 95% confidence intervals for isopleth modeling in Figure 13. <br></div>

Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post- orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

This study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina sug- gest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

Supplemental Material: Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

<div>Text S1: Supplemental text. Figure S1: Cathodoluminescence images for all analyzed zircon grains. Figure S2: REE spider plots for zircon. Figure S3: Tukey honestly significant difference (HSD) for the timing of anatexis. Table S1: Cathodoluminescence images for all analyzed zircon grains. Table S2. Grossular content of garnet used to calculate the 95% confidence intervals for isopleth modeling in Figure 13. <br></div>

Supplemental Material: Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

<div>Text S1: Supplemental text. Figure S1: Cathodoluminescence images for all analyzed zircon grains. Figure S2: REE spider plots for zircon. Figure S3: Tukey honestly significant difference (HSD) for the timing of anatexis. Table S1: Cathodoluminescence images for all analyzed zircon grains. Table S2. Grossular content of garnet used to calculate the 95% confidence intervals for isopleth modeling in Figure 13. <br></div>

Building a continental arc section: Constraints from Paleozoic granulite-facies metamorphism, anatexis, and magmatism in the northern margin of the Qilian Block, northern Tibet Plateau

Continental arcs in active continental margins (especially deep-seated arc magmatism, anatexis, and metamorphism) can be extremely significant in evaluating continent building processes. In this contribution, a Paleozoic continental arc section is constructed based on coeval granulite-facies metamorphism, anatexis, and magmatism on the northern margin of the Qilian Block, which record two significant episodes of continental crust growth. The deeper layer of the lower crust mainly consists of medium-high pressure mafic and felsic granulites, with apparent peak pressure-temperature conditions of 11−13 kbar and 800−950 °C, corresponding to crustal depths of ∼35−45 km. The high-pressure mafic granulite and local garnet-cumulate represent mafic residues via dehydration melting involving breakdown of amphibole with anatectic garnet growth. Zircon U-Pb geochronology indicates that these high-grade metamorphic rocks experienced peak granulite-facies metamorphism at ca. 450 Ma. In the upper layer of the lower crust, the most abundant rocks are preexisting garnet-bearing metasedimentary rocks, orthogneiss, and local garnet amphibolite, which experienced medium-pressure amphibolite-facies to granulite-facies metamorphism at depths of 20−30 km at ca. 450 Ma. These metasedimentary rocks and orthogneiss have also experienced partial melting involving mica and rare amphibole at 457−453 Ma. The shallow to mid-crust is primarily composed of diorite-granodiorite batholiths and volcanic cover with multiple origin, which were intruded during 500−450 Ma, recording long-term crustal growth and differentiation episode. As a whole, two episodes of continental crust growth were depicted in the continental arc section on the northern margin of the Qilian Block, including: (a) the first episode is documented in a lithological assemblage composing of coeval mafic-intermediate intrusive and volcanic rocks derived from partial melting of modified lithospheric mantle and subducted oceanic crust during southward subduction of the North Qilian Ocean at 500−480 Ma; (b) the second episode is recorded in mafic rocks derived from partial melting of modified lithospheric mantle during transition from oceanic subduction to initial collision at 460−450 Ma.