Universal WBC reduction

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75583/1/j.1537-2995.2000.40060751.x.pd

Multidisciplinary approach to study migmatites: origin and tectonic history of the nason ridge migmatitic gneiss, wenatchee block, cascades crystalline core, wa, usa

The Nason Ridge Migmatitic Gneiss of the Cascades Core is a migmatitic unit comprising concordant pelitic schist and gneiss, amphibolite, and tonalite gneiss, and cross cutting tonalite, quartz-rich granitoid, and pegmatite. There are several generations of 'igneous' lithologies (leucosomes = tonalite, quartz-rich granitoid, and pegmatite) some of which are concordant; others clearly crosscut the strongly deformed host rocks. The host rocks are interpreted to be Chiwaukum Schist with metasedimentary (pelitic schist and some gneiss) and metavolcanic(amphibolites) origins. Metamorphic fabric in the Nason Ridge Migmatitic Gneiss is characterized by preferred orientation of platy minerals (continuous schistosity), compositional layering, mineral lineations (elongate grains and grain aggregates), and non-coaxial deformational features (asymmetric augen, grain offsets,rotated porphyroblasts, etc.). Compositional layering is characterized by quartz-plagioclase lenses and patches (mm to cm scale) and by large variations in biotite content. This composite fabric is faulted and folded by mesoscopic structures. The most strongly foliated leucosomes (gneissic tonalites) are generally concordant with the regional trend of foliation, while weakly foliated leucosomes (tonalites) and pegmatite veins crosscut host rock and tonalite gneisses. Thin melanosome layers (biotiteand amphibole schist) are developed locally around quartz - plagioclase lenses and patches. Metamorphism in the Nason Ridge Migmatitic Gneiss and the nearby Chiwaukum Schist likely peaked after intrusion of the Mt. Stuart Batholith ca. 91-94 Ma. Peak temperatures and pressures for the Nason Ridge Migmatitic Gneiss in the Wenatchee Ridge and Pacific Crest areas were 650 - 720 °C and 6 - 9 kbar with a pressure increase of £ 2.0 kbar during metamorphism. Thermodynamic modeling indicates that hydrous partial melting would begin at ca. 660 °C and is relatively pressure independent. Field and petrographic observations, mineral chemistry and thermobarometry, and bulk rock chemistry and thermodynamic modeling of phase equilibria (pseudosections) applied to the Nason Ridge 235Migmatitic Gneiss indicate that at least some of the leucosome bodies were derived by local partial melting. The clearly intrusive character and the sharp contacts between some tonalite leucosome bodies and host rock support an externally derived origin for these tonalite melts. However, some of these bodies may have originated from partial melting of the host Chiwaukum Schist and traveled a short distance before crystallization, or have been modified by deformation so as to obscure textural evidence for local derivation. Results are compatible with derivation of leucosome rocks in the Nason Ridge Migmatitic Gneiss from two non-exclusive processes: partial melting of the host rock and intrusion of externally derived tonalite melts

MULTIDISCIPLINARY APPROACH TO STUDY MIGMATITES: ORIGIN AND TECTONIC HISTORY OF THE NASON RIDGE MIGMATITIC GNEISS, WENATCHEE BLOCK, CASCADES CRYSTALLINE CORE, WA, USA

The Nason Ridge Migmatitic Gneiss of the Cascades Core is a migmatitic unit comprising concordant pelitic schist and gneiss, amphibolite, and tonalite gneiss, and cross cutting tonalite, quartz-rich granitoid, and pegmatite. There are several generations of 'igneous' lithologies (leucosomes = tonalite, quartz-rich granitoid, and pegmatite) some of which are concordant; others clearly crosscut the strongly deformed host rocks. The host rocks are interpreted to be Chiwaukum Schist with metasedimentary (pelitic schist and some gneiss) and metavolcanic(amphibolites) origins. Metamorphic fabric in the Nason Ridge Migmatitic Gneiss is characterized by preferred orientation of platy minerals (continuous schistosity), compositional layering, mineral lineations (elongate grains and grain aggregates), and non-coaxial deformational features (asymmetric augen, grain offsets,rotated porphyroblasts, etc.). Compositional layering is characterized by quartz-plagioclase lenses and patches (mm to cm scale) and by large variations in biotite content. This composite fabric is faulted and folded by mesoscopic structures. The most strongly foliated leucosomes (gneissic tonalites) are generally concordant with the regional trend of foliation, while weakly foliated leucosomes (tonalites) and pegmatite veins crosscut host rock and tonalite gneisses. Thin melanosome layers (biotiteand amphibole schist) are developed locally around quartz - plagioclase lenses and patches. Metamorphism in the Nason Ridge Migmatitic Gneiss and the nearby Chiwaukum Schist likely peaked after intrusion of the Mt. Stuart Batholith ca. 91-94 Ma. Peak temperatures and pressures for the Nason Ridge Migmatitic Gneiss in the Wenatchee Ridge and Pacific Crest areas were 650 - 720 °C and 6 - 9 kbar with a pressure increase of £ 2.0 kbar during metamorphism. Thermodynamic modeling indicates that hydrous partial melting would begin at ca. 660 °C and is relatively pressure independent. Field and petrographic observations, mineral chemistry and thermobarometry, and bulk rock chemistry and thermodynamic modeling of phase equilibria (pseudosections) applied to the Nason Ridge 235Migmatitic Gneiss indicate that at least some of the leucosome bodies were derived by local partial melting. The clearly intrusive character and the sharp contacts between some tonalite leucosome bodies and host rock support an externally derived origin for these tonalite melts. However, some of these bodies may have originated from partial melting of the host Chiwaukum Schist and traveled a short distance before crystallization, or have been modified by deformation so as to obscure textural evidence for local derivation. Results are compatible with derivation of leucosome rocks in the Nason Ridge Migmatitic Gneiss from two non-exclusive processes: partial melting of the host rock and intrusion of externally derived tonalite melts

Prolonged metamorphism during long-lived terrane accretion: Sm-Nd garnet and U-Pb zircon geochronology and pressure-temperature paths from the Salmon River suture zone, west-central Idaho, USA

The Salmon River suture zone of western Idaho (USA) records mid-crustal metamorphism and deformation associated with orogenesis during Mesozoic accretion of volcanic arc terranes to western Laurentia. We present petrographic and microstructural observations, garnet geochemistry, pressure-temperature isochemical phase diagrams, and Sm-Nd garnet and U-Pb zircon ages to investigate the timing and conditions of metamorphism in the Salmon River suture zone. The Salmon River suture zone is comprised of three thrust sheets: from east to west, the amphibolite facies Pollock Mountain plate, upper greenschist to amphibolite facies Rapid River plate, and greenschist facies Heavens Gate plate. The Pollock Mountain plate was isothermally loaded from 6 to \u3e8 kbar at ∼700 °C between 141 and 124 Ma during northwest-southeast crustal shortening. The underlying Rapid River plate was isothermally loaded from 7 to ∼10 kbar at 600-650 °C during ca. 124-112 Ma metamorphism, which is contemporaneous with late- to post-peak metamorphism and ca. 118 Ma exhumation of the overlying Pollock Mountain plate. In the Rapid River plate, thrust sheet emplacement induced high-strain ductile deformation and led to regional development of linear-planar fabrics. The 206Pb/238U zircon ages for syndeformational to postdeformational magmatism record ca. 117 Ma or younger juxtaposition of the two plates on the southeast-dipping Pollock Mountain thrust fault. Coeval 124-112 Ma metamorphism of the Rapid River plate, ca. 118 Ma exhumation of the Pollock Mountain plate, and ca. 117 Ma or younger movement along the Pollock Mountain fault suggest that metamorphism of the Rapid River plate was possibly driven in part by thrust juxtaposition and loading along the Pollock Mountain fault. In this context, we interpret that metamorphism records diachronous thrust stacking during prolonged (\u3e30 m.y.) accretionary orogenesis in western Idaho

Pressure-temperature-time paths, prograde garnet growth, and protolith of tectonites from a polydeformational, polymetamorphic terrane: Salmon River Suture Zone, West-Central Idaho

The metamorphic rocks of the Salmon River suture zone (SRSZ) in west-central Idaho provide a unique glimpse into mid-lower crustal processes during continental growth by island arc accretion. The SRSZ, which separates island arc terranes of the Blue Mountains Province (BMP) from the Mesozoic margin of North America, contains medium to high grade tectonites that record multiple metamorphic and deformation events. The SRSZ is divided by the Pollock Mountain thrust fault (PMtf) into two structural blocks: the higher-grade Pollock Mountain plate (PMp), and the lower-grade, underlying Rapid River plate (RRp). Previous studies interpreted pre-144 Ma metamorphism within the SRSZ related to assembly of the BMP. Counter-clockwise P-T paths for metamorphism within the RRp [peak=8-9 kbar ~600°C, retrograde=5-7 kbar, 450- 525°C] were inferred to include prograde garnet growth during pre-144 Ma loading followed by garnet growth during rapid cooling due to lithospheric delamination. The PMp was interpreted to have subsequently been buried to increasing depth and metamorphosed again at 128 Ma as a result of the BMP docking with North America. New P-T-t paths for the RRp and PMp constructed from geochronology, geothermobarometry, pseudosections, and petrography suggest that after loading, slow cooling rates caused diffusion in garnet rims, which produced counter-clockwise P-T paths. Garnet Sm-Nd ages of 112.5±1.5 Ma from the RRp, and 141-124 Ma from the PMp suggest that metamorphism within the SRSZ is diachronous and that crustal thickening was protracted occurring between 141-112 Ma. P-T-t paths between both plates indicate that the PMp reached peak metamorphism prior to peak metamorphism of the RRp. This suggests that the PMp was buried prior to the development of the PMtf. The RRp was subsequently buried along the PMtf, which was followed by development of the Rapid River thrust fault, which juxtaposed RRp schists onto the Wallowa terrane of the BMP. This model suggests that metamorphism in the SRSZ was controlled by individual thrust faults instead of recording collisions between terranes and is consistent with a prolonged burial of rocks in the SRSZ followed by slow cooling that does not require lithospheric delamination to account for retrograde P-T estimates. (Published By University of Alabama Libraries

Metamorphism of the Wenatchee Ridge orthogneiss: a combined application of geochronology and phase equilibrium modeling

Pressure and temperature estimates from phase diagram models indicate that the Wenatchee Ridge orthogneiss is the deepest exposed segment of the crust in the Nason terrane of the Cascades crystalline core, WA. Peak metamorphic P-T conditions of 8-11.5 kbar and ~600-700 ºC exceed predictions for the Chiwaukum Schist and Mt. Stuart batholith to the west-southwest and are compatible with a gradient of increasing P and T from the southernmost Nason terrane to the NRMG and WRO. Wenatchee Ridge orthogneiss zircon U-Pb ages (88.9-84.8 Ma) are identical to other zircon ages, interpreted as ages of intrusion, from the Pear Lake orthogneiss. Additional similarities in external grain morphology and internal zoning suggest an igneous origin to the Wenatchee Ridge orthogneiss. The P-T results and ages support the interpretation of the Wenatchee Ridge orthogneiss origin as one of many intrusive bodies within the Nason Ridge Migmatitic Gneiss, all of which intruded the Chiwaukum Schist. In addition, the results are compatible with a proposed southwest oriented thrust loading resulting in 1) older metamorphic ages and 2) elevated pressure and temperature conditions in the northeastern Nason terrane. (Published By University of Alabama Libraries

U-Pb zircon and monazite geochronology and hafnium isotopic geochemistry of neoacadian and early alleghanian plutonic rocks in the Alabama Eastern Blue Ridge, Southern Appalachian Mountains

The Alabama eastern Blue Ridge (EBR) of the Southern Appalachian Mountains hosts a variety of felsic plutonic rocks, which intrude multiply deformed Neoproterozoic to Ordovician metasedimentary rocks. Plutons consist of two distinct suites based on geochemical composition and degree of deformation: pre- to syn-kinematic Neoacadian, low Sr/Y plutons (ca. 380-360 Ma) and late- to post-kinematic, Early Alleghanian high Sr/Y plutons (ca. 350-330 Ma). Here, I report new whole rock geochemistry, U-Pb zircon SHRIMP-RG (Sensitive High Resolution Ion Micro Probe-Reverse Geometry) ages, and Hf isotope data for 6 plutons in the Alabama EBR. Low Sr/Y plutons are predominantly biotite-muscovite granites and granodiorites and include the Rockford Granite (376.6 ± 1.5 Ma) and the Bluff Springs Granite (363.8 ± 2.9 Ma). The Enitachopco trondhjemite dike also displays a Neoacadian age of 366.5 ± 3.5 Ma. Zircon Hf isotope data from the low Sr/Y suite range from -11.2 to +2.0. These plutons are in general strongly deformed, and display geochemical characteristics consistent with mid crustal (<35 km) partial melting of pre-existing continental crust. By contrast, high Sr/Y plutons are deformed to undeformed, and consist of low-K tonalites and trondhjemites (e.g., Almond trondhjemites and Blakes Ferry pluton) with geochemical characteristics suggestive of deep-crustal partial melting of a garnet amphibole-bearing source. Two samples of the Almond trondhjemite (Wedowee pluton and Almond pluton) yielded ages of 334.6 ± 3.2 Ma and 343.4 ± 3.4 Ma, respectively. An additional peak at 324.4 ± 3.3 may represent a Pb-loss event. Another sample of Almond trondhjemite yielded complex ages with a peak at 349.1 ± 1.8 Ma The undeformed Blakes Ferry pluton also yielded complex results with Grenville-age cores (ca. 1000-1080 Ma), and rim ages ranging from ca. 350 to 330 Ma with peaks at 343.1 ± 3.3 Ma and 331.1 ± 3.8. Igneous monazite yielded an age of 345.9 ± 3.1 Ma supporting a ca. 345 Ma crystallization age. Hf isotope data from the high Sr/Y suite range from -14.6 to +5.6. We propose that the transition from Neoacadian, low Sr/Y, mid-crustal partial melting to Early Alleghanian high Sr/Y deep crustal partial melting reflects thickening of the EBR during Neoacadian deformation. Hf isotope values also transition from crustal values (-εHf) to a mixed signature (+εHf and -εHf), reflecting both mantle and lower crustal melting. This transition may be related to slab break off following Neoacadian deformation. (Published By University of Alabama Libraries

Explaining discontinuous garnet zoning using reaction history p-t models: an example from the Salmon River suture zone, west-central Idaho

Discontinuously zoned or two-stage garnet has been observed in numerous locations and geologic settings worldwide. These garnets are characterized by sharp breaks in inclusion density and compositional zoning, and often, these sharp breaks are interpreted as a hiatus in growth, change in growth rate, change in bulk rock composition, chemical diffusion, or absorption and new growth of garnet. During accretion of terranes and microplates, thermal pulses and thrust fault movements occur, which drive metamorphism and therefore the growth of garnet. Multiple garnet growth events could produce a discontinuously zoned garnet and each growth stage could be interpreted to represent a separate metamorphic event. Two-stage garnet is common in the Salmon River suture zone (SRSZ) and multiple tectonic models have been proposed based on the two-stage garnet. Getty et al. (1993) and Selverstone et al. (1992) proposed multiple accretion and metamorphic events based on the estimates for pressure, temperature, and age of these garnets. Recently, McKay (2011) proposed that heating after several major fault displacements caused the growth of two-stage garnet. This study uses compositions of garnet cores and rims on isochemical phase diagrams to construct new garnet growth P-T paths. Core and rim P-T estimates combined with observed mineral assemblages indicate an initial garnet growth reaction, followed by a reaction consuming and then growing garnet, e.g., chlorite + garnet = amphibole + H2O and amphibole = garnet + Al2SiO5 (kyanite) + H2O. Isochemical P-T modeling of garnet modal percentages, mineral compositions, and petrologic observations supports the occurrence of these reactions in the SRSZ garnet. The proposed reaction history would produce two-stage garnet along a single prograde path, which does not require multiple thermal and tectonic events. This interpretation supports the single terrane accretion hypothesis proposed by McKay (2011). (Published By University of Alabama Libraries

U-Pb zircon geochronology, Hf isotope and trace element geochemistry of a unique lower crustal - upper mantle section of a dying slow-spreading mid-ocean ridge (Macquarie Island, Southern Ocean)

Macquarie Island, located in the Southern Ocean, is a section of subaerially exposed oceanic crust formed at the now extinct proto-Macquarie, slow-spreading, mid-ocean ridge. Macquarie Island is unique among oceanic ophiolite sequences in that it is still located in the basin in which it formed. The northernmost part of Macquarie Island is composed primarily of lower crustal gabbro and upper mantle peridotite, and therefore provides a unique window into lower oceanic crust. Here, we report integrated Pb/U zircon ages and Lu-Hf isotopic and trace element data from six samples of the lower crust-upper mantle sequence. Samples consist of two lower crustal gabbros, and three mantle-hosted gabbro dikes/dikelettes and one phlogopite-bearing vein from the upper mantle sequence. ^206 Pb/^238 U SHRIMP-RG zircon error weighted average ages for the lower crustal gabbros are 8.7 ± 0.3 Ma (N = 9) and 9.0 ± 0.2 Ma (N = 13), whereas the mantle-hosted gabbro dikes/dikelettes yield overlapping error-weighted average ages of 8.7 ± 0.2 Ma (N = 9), 8.7 ± 0.3 Ma (N = 8), and 8.9 ± 0.2 Ma (N = 11) (all errors 2σ). The phlogopite vein yielded a slightly younger error weighted average age of 8.5 ± 0.1 Ma (N = 11). Initial epsilon Hf results for zircons from the same samples show a broad distribution ranging from +9.5 to +13.3 for the lower crustal gabbro (N = 28), +7.0 to +16.4 for the gabbro dikes/dikelettes (N = 24), and +8.4 to +12.2 for the phlogopite vein (N = 12). The wide range in values (particularly from the gabbro dikes/dikelettes) is consistent with a heterogeneous source region composed of depleted- and enriched mantle sources. Zircon trace element concentrations also support a heterogeneous source, displaying enrichment in U/Yb relative to N-MORB zircons from the Mid-Atlantic and Southwest Indian Ridge systems. No pattern of enrichment with time is observed within the resolution of the U-Pb zircon dates. We interpret these results to indicate that magmatic construction of Macquarie Island occurred between 8.7 and 9.0 Ma and involved sampling of at least two distinct mantle sources. (Published By University of Alabama Libraries