HT/HP metamorphism and crustal anatexis.

Early mafic magmatism and crustal anatexis on the Isle of Rum: evidence from the Am Mam intrusion breccia

by Eoghan Holohan

Nicoll, G.R., Holness, M.B., Troll, V.R., Donaldson, C.H., Holohan, E.P., Emeleus C.H., & Chew D.
Published in Geological Magazine (2009)

The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern... more The Rum Igneous Centre comprises two early marginal felsic complexes (the Northern Marginal Zone and the Southern Mountains Zone), along with the later central ultrabasic–basic layered intrusions. These marginal complexes represent the remnants of near-surface to eruptive felsic magmatism associated with caldera collapse, examples of which are rare in the North Atlantic Igneous Province. Rock units include intra-caldera collapse breccias, rhyolitic ignimbrite deposits and shallow-level felsic intrusions, as well the enigmatic ‘Am Màm intrusion breccia’. The latter comprises a dacitic matrix enclosing lobate basaltic inclusions (~1–15 cm) and a variety of clasts, ranging from millimetres to tens of metres in diameter. These clasts comprise Lewisian gneiss, Torridonian sandstone and coarse gabbro. Detailed re-mapping of the Am Màm intrusion breccia has shown its timing of emplacement as syn-caldera, rather than pre-caldera as previously thought. Textural analysis of entrained clasts and adjacent, uplifted country rocks has revealed their thermal metamorphism by early mafic intrusions at greater depth than their present structural position. These findings provide a window into the evolution of the early mafic magmas responsible for driving felsic magmatism on Rum. Our data help constrain some of the physical parameters of this early magma–crust interaction and place it within the geochemical evolution of the Rum Centre.

Cretaceous oblique detachment tectonics in the Fosdick Mountains, Marie Byrd Land, Antarctica

by Seth Kruckenberg

McFadden, R., Siddoway, C.S., Teyssier, C., Fanning, C.M., and Kruckenberg, S.C. (2007), Cretaceous oblique detachment tectonics in the Fosdick Mountains, Marie Byrd Land, Antarctica, in: Antarctica: A Keystone in a Changing World-Online Proceedings of the 10th ISAES, edited by A.K. Cooper and C.R. Raymond et al, USGS Open-File Report 2007-1047, Short Research Paper 046, 6 p. doi: 103133/of2007-1047.srp046.

The Fosdick Mountains form an E-W trending migmatite dome in the northern Ford Ranges of Marie Byrd Land, Antarctica.... more The Fosdick Mountains form an E-W trending migmatite dome in the northern Ford Ranges of Marie Byrd Land, Antarctica. Pervasively folded migmatites derived from lower Paleozoic greywacke and middle Paleozoic plutonic rocks constitute the dome. New field research documents a transition from melt-present to solid-state deformation across the south flank of the dome, and a mylonitic shear zone mapped for 30 km between Mt. Iphigene and Mt Richardson. Kinematic shear sense is dextral normal oblique, with top-to-the-SW and -WSW transport. A U-Pb age of 107 Ma, from a leucosome-filled extensional shear band, provides a melt present deformation age, and a U-Pb age of 96 Ma, from a crosscutting granitic dike, gives a lower age limit for deformation. The shear zone, here named the South Fosdick detachment zone, forms the south flank of the migmatite dome and was in part responsible for the exhumation of mid-crustal rocks.

Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece

by Seth Kruckenberg

Kruckenberg, Seth C., Vanderhaeghe, Olivier, Ferré, Eric C., Teyssier, Christian, and Whitney, Donna L., (2011), Flow of partially molten crust and the internal dynamics of a migmatite dome, Naxos, Greece, Tectonics, v. 30, TC3001, 24 pp., doi: 10.1029/2010TC002751

Migmatite domes are common in metamorphic core complexes. Dome migmatites deform in the partially molten or magmatic... more Migmatite domes are common in metamorphic core complexes. Dome migmatites deform in the partially molten or magmatic state and commonly record complex form surfaces, folds, and fabrics while units mantling the dome display a simpler geometry, typically formed by transposition during crustal extension. We use field observations and magnetic fabrics in the Naxos dome (Greece) to quantify the complex flow of anatectic crust beneath an extensional detachment system. The internal structure of the Naxos dome is characterized by second-order domes (subdomes), pinched synforms, and curved lineation trajectories, which suggest that buoyancy-driven flow participated in dome evolution. Subdomes broadly occur within two compartments that are separated by a steep, N-S oriented, high-strain zone. This pattern has been recognized in domes formed by polydiapirism and in models of isostasy-dominated flow. The preferred model involves a combination of buoyancy- and isostasy-driven processes: the Naxos dome may have been generated by regional N-S extension that triggered convergent flow of partially molten crust at depth and the upwelling of anatectic migmatites within the dome. This pattern is complicated by gravitational instabilities and/or overturning of the high melt fraction crust leading to the growth of subdomes. As the migmatites within the Naxos dome reached a higher structural level, they were affected by regional top-to-the-NNE kinematics of the detachment system. Dome formation therefore occurred by a combination of coeval and coupled processes: upper crustal extension, deep crust contraction during convergent flow of anatectic crust, diapirism and/or density-driven crustal convection forming subdomes, and north directed detachment kinematics.

Viscous collision in channel explains double domes in metamorphic core complexes

by Seth Kruckenberg

Rey, Patrice F., Teyssier, Christian, Kruckenberg, Seth C., Whitney, Donna L. (2011), Viscous collision in channel explains double domes in metamorphic core complexes, Geology, v. 39, no. 4, p. 387-390, doi: 10.1130/G31587.1

In hot orogens, gneiss domes are a response to upper crustal stretching and lower crustal flow. Two-dimensional... more In hot orogens, gneiss domes are a response to upper crustal stretching and lower crustal flow. Two-dimensional thermal-mechanical modeling shows that localization of extension in the upper crust triggers, in the deep crust, oppositely verging horizontal flows that converge beneath the extended region. Upon viscous collision, both flowing regions rotate upward to form two upright domes of foliation (double domes) separated by a steep median high-strain zone. In such systems, horizontal shortening in the infrastructure develops in an overall extensional setting. Dome material follows a complex depth-dependent strain history, from shearing in the deep crustal channel, to contraction upon viscous collision in the median high-strain zone, to extension upon advection into the shallow crust. This depth-dependent strain history is likely a general feature of dome evolution, and is arguably well preserved in double domes such as the Montagne Noire (France) and Naxos (Greece) gneiss domes.

Viscoplastic flow in migmatites deduced from fabric anisotropy: An example from the Naxos dome, Greece

by Seth Kruckenberg

Kruckenberg, Seth C., Ferré, Eric C., Teyssier, Christian, Vanderhaeghe, Olivier, Whitney, Donna L., Seaton, Nicholas C.A., and Skord, Justin A. (2010), Viscoplastic flow in migmatites deduced from fabric anisotropy: an example from the Naxos dome, Greece, Journal of Geophysical Research - Solid Earth, v. 115, B09401, doi: 10.1029/2009JB007012.

Many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of... more Many migmatites represent crystallized partially molten crust and therefore record the mechanisms and pathways of orogenic crustal flow. Field and microstructural methods may be insufficient to characterize the planar and linear elements of rock fabric in migmatites due to obscured flow fabrics or protracted deformation. In the Naxos dome (Greece), we test the anisotropy of magnetic susceptibility (AMS) as a tool for recovering mineral fabric symmetry and the kinematic axes of flow in migmatites. Measurements of 155 migmatite samples yield dominantly low values (<300 × 10−6 [SI]) of bulk magnetic susceptibility (Km) consistent with biotite being the dominant carrier of the AMS. Higher values of Km, thermomagnetic, hysteresis, and microstructural data, however, suggest a ferromagnetic contribution from magnetite in a subset of samples (N = 15). Using electron backscatter diffraction (EBSD) analysis, we establish the correspondence of the biotite subfabric with the AMS and structural fabric of the Naxos migmatites. EBSD data from biotite suggests that magnetic lineation in these dominantly paramagnetic migmatites arises from a zone axis orientation of biotite crystals organized about the direction of viscoplastic flow. Over a range of spatial scales, migmatitic foliation and magnetic foliation are well correlated. The magnetic lineation recovered by AMS displays a coherent organization despite the heterogeneous structure and composition of the Naxos migmatites. These data suggest that the apparent complexity of migmatites masks a simpler flow regime controlled by bulk viscoplastic flow. Furthermore, our study demonstrates the utility of the AMS method for studying the dynamics of partially molten orogenic crust.

Paleocene-Eocene migmatite crystallization, extension, and exhumation in the hinterland of the northern Cordillera: Okanogan dome, Washington, USA

by Seth Kruckenberg

Kruckenberg, Seth C., Whitney, Donna L., Teyssier, Christian, Fanning, Mark, and Dunlap, W.James. (2008), Paleocene-Eocene migmatite crystallization, extension, and exhumation in the hinterland of the Northern Cordillera: Okanogan dome, Washington, USA, Geological Society of America Bulletin, v. 120, p. 912-929. doi: 10.1130/B26153.1.

The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a... more The Okanogan gneiss dome, Washington, is located in the hinterland of the North American Cordillera and is part of a chain of metamorphic core complexes containing gneiss and migmatite domes exhumed during Eocene extension of thickened crust. U-Pb sensitive high-resolution ion micro-probe (SHRIMP) analyses of zircon, monazite, and titanite, and 40Ar-39Ar analyses of biotite from migmatites exposed in the footwall of the Okanogan detachment, coupled with a detailed structural analysis, document the timing and duration of migmatite crystallization and indicate coeval crystallization, extensional deformation, and exhumation of the dome. Okanogan migmatites are folded and deformed, and preserve successive generations of leucosomes generated by synkinematic anatexis.

Analyses of migmatite samples from a high-melt fraction subdome near Stowe Mountain suggest that the Okanogan dome records a history of migmatite crystallization spanning at least 12 m.y., as indicated by 206Pb/238U ages ranging from ca. 61 to 49 Ma for new zircon growth and rim overgrowths attributed to migmatite crystallization. Zircons from a granodiorite in a domain of diatexite near Stowe Mountain preserve rims that have a mean 206Pb/238U age of 51.1 ± 1.0 Ma for the youngest population attributed to migmatite crystallization. Zircon from folded and discordant granitic leucosome in the diatexite domain yields a calculated 206Pb/238U age of 53.5 ± 0.5 Ma for migmatite crystallization. Zircon from discordant leucosome of the metatexite domain has a mean 206Pb/238U age of 59.8 ± 0.5 Ma, with ages as young as ca. 53 Ma attributed to final crystallization of the leucosome. Core domains of zircon samples have 206Pb/238U ages that range from ca. 85 to 70 Ma and are interpreted to be related to an earlier phase of the orogeny. Monazite from two samples gives 206Pb/238U crystallization ages of 52.9 ± 0.6 Ma for the granodiorite diatexite and 52.0 ± 0.6 Ma for nearby boudinaged and foliated layers of biotite granodiorite. One sample of folded granitic leucosome in metatexite contains titanite with a mean 206Pb/238U age of 47.1 ± 0.5 Ma. The ca. 47 Ma age for titanite is similar to biotite 40Ar-39Ar ages of 48.0 ± 0.1 Ma, 47.9 ± 0.2 Ma, and 47.1 ± 0.2 Ma for samples collected from the upper detachment surface downward over 1.5 km of structural thickness into the migmatite domain.

Crystallization of the Okanogan migmatites was therefore coeval in part with upper crustal extension and ductile flow of the mid-crust. Leucosome crystallization largely ceased by ca. 49 Ma, followed by rapid cooling of footwall rocks through ~325 °C by ca. 47 Ma. These data are similar to crystallization ages in migmatites from other domes in the northern Cordillera hinterland, suggesting that crustal anatexis was widespread over much of the mid-crust during Paleocene to Eocene time, coeval with extension and exhumation of orogenic middle crust.

Metamorphic evolution of sapphirine‐and orthoamphibole‐cordierite‐bearing gneiss, Okanogan dome, Washington, USA

by Seth Kruckenberg

Kruckenberg, Seth C., and Whitney, Donna L. (2011), Metamorphic evolution of sapphirine- and orthoamphibole-cordierite-bearing gneiss, Okanogan dome, Washington, USA, Journal of Metamorphic Geology. v. 29 (4), p. 425-449, doi: 10.1111/j.1525-1314.2010.00926.x

Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the... more Gneiss domes are commonly cored by quartzofeldspathic rocks that provide little information about the pressure–temperature–fluid history of the domes. Three northern Cordilleran migmatite domes (Thor-Odin and Valhalla/Passmore, British Columbia, Canada; Okanogan, Washington, USA), however, contain Mg–Al-rich orthoamphibole-cordierite gneiss as layers and lenses that record metamorphic conditions and pressure–temperature (P–T) path information not preserved in the host migmatite. These Mg–Al-rich rocks are therefore a valuable archive of metamorphic conditions during dome evolution, although refractory rocks such as these commonly contain reaction textures that may complicate the calculation of metamorphic conditions. In the Okanogan dome, Mg–Al-rich layers are part of the Tunk Creek unit, which occurs at the periphery of an underlying migmatite domain. Bulk compositional layers (mm- to m-scale) consist of gedrite-dominated, hornblende-dominated and biotite-bearing layers that contain variable amounts of gedrite, hornblende, anorthite, cordierite, spinel, sapphirine, corundum, kyanite, biotite and/or staurolite. The presence of different compositional layers (some with reaction textures, some without) allows systematic analysis of metamorphic history by a combined petrographic and phase equilibrium analysis. Gedrite-dominated layers containing relict kyanite preserve evidence of the highest-P conditions; symplectitic and coronal reaction textures around kyanite indicate decompression at high temperature. Gedrite-dominated layers lacking these reaction textures contain layers of sapphirine and spinel in apparent textural equilibrium and record a later high-T–low-P part of the path. Phase equilibria (pseudosection) analysis for layers that lack reaction textures indicates metamorphic conditions of 720–750 °C at a range of pressures (>8 to <4 kbar) following decompression. Elevated crustal temperatures and concordant structural fabrics in the Tunk Creek unit and underlying migmatite domain suggest that the calculated P–T conditions recorded in Tunk Creek rocks were coeval with anatexis, extension, and dome formation in Palaeocene–Eocene time. In contrast to orthoamphibole-cordierite gneiss in the other Cordilleran domes, the Tunk Creek unit occurs as a discontinuous km-scale layer rather than as smaller (m-scale) pods, is more calcic, and lacks garnet. In addition, kyanite did not transform to sillimanite, and spinel commonly occurs as a blocky matrix phase in addition to vermicules in symplectite. These differences, along with the compositional layering, allow an analysis of bulk composition v. tectonic (P–T path) controls on mineral assemblages and textures. Pseudosection modelling of different layers in the Tunk Creek unit provides a basis for understanding the metamorphic history of these texturally complex, refractory rocks and their host gneiss domes, and other such rocks in similar tectonic settings.

A new occurrence of microdiamond-bearing metamorphic rocks, SW Rhodopes, Greece

by Silke Schmidt

Schmidt, S., Nagel, T.J., Froitzheim, N. (2010) - European Journal of Mineralogy 22, 189-198, doi: 10.1127/0935-1221/2010/0022-1999.

Magma ascent along a major terrane boundary: crustal contamination and magma mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland.

by Fiona Meade

Meade, F.C., Chew, D.M., Troll, V.R., Ellam, R.M., Page, L.M. (2009). Magma ascent along a major terrane boundary: crustal contamination and magma mixing at the Drumadoon Intrusive Complex, Isle of Arran, Scotland. Journal of Petrology., v.50 (12), 2345-2374. http://petrology.oxfordjournals.org/content/50/12/2345

The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form... more The composite intrusions of Drumadoon and An Cumhann crop out on the SE coast of the Isle of Arran, Scotland and form part of the larger British and Irish Palaeogene Igneous Province, a subset of the North Atlantic Igneous Province. The intrusions (shallow-level dykes and sills) comprise a central quartz–feldspar-phyric rhyolite flanked by xenocryst-bearing basaltic andesite, with an intermediate zone of dark quartz–feldspar-phyric dacite. New geochemical data provide information on the evolution of the component magmas and their relationships with each other, as well as their interaction with the crust through which they travelled. During shallow-crustal emplacement, the end-member magmas mixed. Isotopic evidence shows that both magmas were contaminated by the crust prior to mixing; the basaltic andesite magma preserves some evidence of contamination within the lower crust, whereas the rhyolite mainly records upper-crustal contamination. The Highland Boundary Fault divides Arran into two distinct terranes, the Neoproterozoic to Early Palaeozoic Grampian Terrane to the north and the Palaeozoic Midland Valley Terrane to the south. The Drumadoon Complex lies within the Midland Valley Terrane but its isotopic signatures indicate almost exclusive involvement of Grampian Terrane crust. Therefore, although the magmas originated at depth on the northern side of the Highland Boundary Fault, they have crossed this boundary during their evolution, probably just prior to emplacement.