Mount Stuart Batholith Research Articles

Pre-Late Wisconsin Valley-Glacier Erratics between Leavenworth and Peshastin, Wenatchee Valley, Washington

The late Wisconsin Icicle Creek alpine glacier transported tonalite boulders from the Mount Stuart batholith to arcuate end moraines in Icicle valley and Wenatchee valley near Leavenworth. Some previous workers considered sparsely weathered Mount Stuart boulders lying outside these moraines and draped by silt as having been ice rafted in a late Wisconsin lake. But the boulders are more likely within drift or end moraines of pre-late Wisconsin glaciations correlative to lateral moraines on Boundary Butte. The silt—a younger deposit that covers the boulders—derives not from a physically ponded lake but apparently from brief Missoula flood(s) backflooding up the valley.

Post-emplacement fluids and pluton thermobarometry: Mount Stuart batholith, Washington Cascades

The effects of post-emplacement infiltration of externally derived, high-temperature fluids into arc-related batholiths are often not well characterized. Such infiltration can have far-reaching effects on the elemental and light isotopic chemistry of a batholith and on its mineral phases. At high temperature, fluid infiltration can be less easily detected, especially if widespread. The Mount Stuart batholith of the Washington Cascades is offered as an example of high-temperature infiltration of high δ18O fluids derived from its contact aureole. Some of the fluid infiltration coincided with and may have been partly derived from a kyanite-grade, post-emplacement metamorphic event that affected northern portions of the batholith. However, the effects of the fluid infiltration were far reaching and affected the entire margin of the batholith, including southerly portions that did not experience post-emplacement metamorphism. The result led to an oxygen isotopic zonation of the batholith, which is viewed as secondary in origin, with expected effects on mineral chemistry, including derived estimates of thermobarometry, a portion of which is also substantially subsolidus in origin. Our revised emplacement barometry of the Mount Stuart batholith, excluding areas affected by fluid infiltration, demonstrates that it was emplaced at ∼350–400 MPa. Soon after emplacement, the batholith was tilted to the north by loading processes and subsequently was righted and unroofed during erosion in the Eocene. Its current near palaeohorizontal orientation has implications for palaeomagnetic studies supporting northward transport (the Baja–British Columbia hypothesis), but these results need further study, given the batholith's complex metamorphic and deformation history and the nature of its dominant magnetic mineralogy (pyrrhotite).

Garnet growth during crustal thickening in the Cascades Crystalline Core, Washington, USA

Garnet Sm–Nd and zircon U–Pb ages, and pressure–temperature–time paths elucidate Late Cretaceous crustal thickening which occurred within magmatic arc rocks of the Insular Superterrane. Voluminous tonalitic magma of the Mount Stuart batholith intruded at <3 kbar into upper crustal sedimentary rocks between 96 and 91 Ma, with initial intrusion prior to garnet growth in the metasedimentary rocks. Arc plutonism then shifted northward as crustal thickening commenced. Initial garnet growth, locally with kyanite and staurolite replacing andalusite, at c. 91 Ma was directly associated with intrusion of granodiorite to tonalite sheets at 7 kbar, north of the Mount Stuart batholith, within the Nason Ridge Migmatitic Gneiss. Subsequent heating and garnet growth, which postdates emplacement of large plutons, occurred between 88 and 86 Ma. This late garnet growth occurred at pressures of 6–8 kbar. The history of garnet growth and intrusion indicates that initial garnet zone and higher temperature metamorphism was restricted to contact aureoles. However, later widespread garnet growth at higher pressure probably resulted from heating as the orogenic wedge approached thermal equilibrium after crustal thickening. We conclude that metasedimentary rocks outside narrow contact aureoles remained at temperatures significantly below those of garnet growth and that the growth of garnet lasted <6 Myr. Heating to temperatures that stabilized garnet after pluton emplacement is compatible with intrusion of arc plutons into an accretionary wedge (Chiwaukum Schist) which was tectonically thickened and/or overthrust causing loading and thermal relaxation.

Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intrusions, North Cascades, Washington

Deciphering the magmatic history of continental magmatic arcs, in general, and the growth history of individual intrusions, in particular, is key to understanding the complex history of magma generation, segregation, and transport that define the dynamics of crustal growth. We utilize high precision U-Pb geochronology to resolve a detailed magmatic history from two composite intrusions, the 2–4 kbar Mount Stuart batholith and the 7–10 kbar Tenpeak pluton, emplaced in the Cretaceous North Cascades arc. This temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts. U-Pb zircon crystallization ages were obtained from 12 samples of the Mount Stuart batholith and 8 samples of the Tenpeak intrusion, representing the range of compositional diversity and geographical extent. These dates indicate that the Mount Stuart batholith was constructed over a ∼5.5 m.y. time period that was punctuated by four intervals of high magma flux. The durations of the high-flux periods are short (a few hundred thousand years) relative to the duration of the batholith. The consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt-rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time-transgressive magma fabrics. In contrast, the Tenpeak intrusion was constructed over a ∼2.6 m.y. time period, with magma influx distributed throughout the intrusive history and texturally distinct magma bodies. The Tenpeak intrusion lacks distinct age domains, which suggests that any magma reservoir was smaller in size and potentially more ephemeral. Although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally, neither pluton bears resemblance to a purely end-member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little, if any, interaction. In particular, ages from the youngest domain of the Mount Stuart batholith indicate that a melt-rich magma reservoir of ≥520 km 3 existed over a 170 ± 90 k.y. time span.

Fast strain rates during pluton emplacement: Magmatically folded leucocratic dikes in aureoles of the Mount Stuart Batholith, Washington, and the Tuolumne Intrusive Suite, California

Finite strain analysis and thermal modeling of magmatically folded leucocratic dikes in the Mount Stuart Batholith, Washington and the Tuolumne Intrusive Suite, California, yield strain rates in the range of 10 - 2 to 10 - 1 3 s - 1 . Compared to published regional strain rates (10 - 1 3 to 10 - 1 5 s - 1 ), wall-rock strain rates associated with dike-fed expansion (10 - 7 s - 1 ), and rates resulting from numerical modeling of crystal-plastic creep in aureoles (10 - 1 1 s - 1 ), our fast rates are several orders of magnitude higher. Field and microstructural observations suggest that multiple material transfer processes, including rigid rotation, ductile flow, cracking, and potentially melt-assisted granular flow operated in the aureoles to accommodate emplacement of these two plutons. Calculated durations of pluton construction and bulk shortening in the aureoles indicate that aureole deformation requires only slow, long-term strain rates of 10 - 1 4 s - 1 . Thus our local fast strain rates indicate that aureoles may be characterized by pulsating high strain rate surges. We suggest that magmatically folded dikes in these and other pluton aureoles around the world may be used as evidence for fast host rock strain rates during pluton emplacement.

Paleomagnetism of the Mount Stuart batholith revisited again: What has been learned since 1972?

<h3>Objective</h3> To assess the prognostic role of serum neurofilament light chains (NfL) for clinically defined multiple sclerosis (CDMS) and McDonald 2017 multiple sclerosis (MS) in patients with clinically isolated syndromes (CIS). <h3>Methods</h3> We retrospectively analyzed data of patients admitted to our neurologic department between 2000 and 2015 for a first demyelinating event. We evaluated baseline serum NfL in addition to CSF, MRI, and clinical data. <h3>Results</h3> Among 222 patients who were enrolled (mean follow-up 100.6 months), 45 patients (20%) developed CDMS and 141 patients (63.5%) developed 2017 MS at 2 years. Serum NfL (median 22.0, interquartile range 11.6–40.4 pg/mL) was noticeably increased in patients with a recent relapse, with a high number of T2 and gadolinium-enhancing lesions at baseline MRI. Serum NfL was prognostic for both CDMS and McDonald 2017 MS, with a threefold and a twofold respective reduction in CDMS and 2017 MS risk in those patients with low and extremely low levels of NfL. The results remained unchanged subsequent to adjustment for such established MS prognostic factors as oligoclonal bands, Gd-enhancing lesions, and a high T2 lesion load at baseline MRI. NfL was associated with disability at baseline but not at follow-up. <h3>Conclusions</h3> Serum NfL have a prognostic value for CIS patient conversion to MS. NfL might play a twin role as biomarker in MS as peak level measurements can act as a quantitative marker of serious inflammatory activity, while steady-state levels can be a reflection of neurodegenerative and chronic inflammatory processes.

Kinetic control of metamorphic imprint during synplutonic loading of batholiths: An example from Mount Stuart, Washington

The superimposition of Barrovian-type metamorphism on low-pressure contact aureoles in magmatic arcs is subject to the kinetic constraints of temperature and influx of H 2 O and may be cryptically developed or not at all. The spatial distribution of detectable overprint will reflect these kinetic factors rather than the geometry of the imposed load. Many of the features believed indicative of magma loading, including apparent steep baric gradients, may be produced in this way. On the basis of adding areas of cryptic metamorphism to the area of obvious Barrovian overprint on the northeast side of the Mount Stuart batholith, we conclude that loading 2–3 m.y. after emplacement was probably much more extensive (sheetlike) than believed previously.

Stoped blocks in plutons: paleo-plumb bobs, viscometers, or chronometers?

Stoped blocks potentially provide information about paleohorizontal during emplacement of plutons, and about the timing of, and viscosities and kinematics during magmatic fabric formation. In the Mount Stuart batholith, Washington, we have examined magmatic fabrics patterns around stoped blocks in tonalite located near the pluton roof and completed 1:1 scale, three-dimensional mapping of fabric patterns around stoped blocks in diorite located ∼1000 m below the roof. In both settings, no structural record of magma ascent, early emplacement, or settling of the blocks is preserved. Instead, fabric formation largely post-dates the settling and trapping of stoped blocks. Two-dimensional finite difference thermal modeling and calculations of sinking rates support short (<2 ×10 3 years) residence times in the magma chamber for these blocks. Relatively symmetric foliation deflections around blocks indicate largely coaxial flattening during fabric formation and that magma viscosity must be lower than the viscosity of the weakest block (metapsammite) but high enough to trap the most dense block (dunite). Block settling calculations indicate that effective magma viscosities were from 10 14 to 10 15 Pa s and/or that yield strengths were large, which suggests that preserved fabrics formed in crystal-rich magmas near the tonalite and diorite solidi. Thus in the Mount Stuart batholith, stoped blocks fail to record information about paleohorizontal, but do indicate that the magmatic fabrics formed after chamber construction, during coaxial strain, and in crystal-rich magma near its solidus.

Magma emplacement during arc‐perpendicular shortening: An example from the Cascades crystalline core, Washington

It is increasingly inferred by others that active arcs at all crustal levels are undergoing arc‐perpendicular extension and that magma utilizes this extension or extension along arc‐parallel, strike‐slip faults during emplacement. However, a review of stress fields in modern arcs and deformation which occurred during magmatism in ancient arcs and our own emplacement studies of individual plutons indicate that magma emplacement typically occurred during arc‐perpendicular shortening without the aid of strike‐slip faulting. This conclusion raises the question of how host rock is displaced during magma emplacement in shortening and thickening orogens. Evaluation of host rock material transfer processes that operated during emplacement of the 93 Ma, 10‐ to 14‐km‐deep Mount Stuart batholith, Washington, provides one example and includes the following: (1) ductile downward flow in a narrow (&lt; 0.2 body radii) structural aureole, (2) stoping, and, of lesser importance, (3) regional folding, thrusting, and subhorizontal arc‐parallel extension. We see no evidence that arc‐perpendicular extension or local extension along strike‐slip faults made space for this pluton nor for most other plutons in contractional arcs. Instead we argue that both localized and rapid (around plutons) and regional and slower downward flow during growth of crustal roots is a common process in such arcs.

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and reply

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and reply

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and Reply

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia: Discussion and Reply

Regional tilt of the Mount Stuart batholith, Washington, determined using aluminum-in-hornblende barometry: Implications for northward translation of Baja British Columbia

We have developed a new quantitative method to estimate paleohorizontal in granitic plutons using the aluminum-in-hornblende (AH) barometer. The method is used to correct previously published paleomagnetic data from the 93‐96 Ma Mount Stuart batholith of the Cascades Mountains, Washington State, for the effects of postemplacement tilting. AH barometry was done on 46 samples from the batholith using the compositions of hornblende rims coexisting with the full mineral assemblage required for pressureestimation.High-contrastback-scatteredelectronimagingwas usedtoensurethattheanalyzedhornblendeswerenotsignificantly affected by subsolidus alteration. The success of the AH barometry is indicated by two observations. First, increases in the Al content of the hornblendes are governed almost entirely by a pressure-sensitive tschermak-type substitution. Second, amphibole-plagioclase thermometry indicates that assemblage equilibration occurred at or very near magmatic conditions (’650 &C) and that temperature has a negligible effect on our pressure estimates. AH barometry results indicate that the depth of crystallization across the batholith decreases systematically from ’0.3 GPa in the northwest to ’0.15 GPa in the southeast,consistentwithindependentbarometryforthecontactaureole ofthebatholithandregionalstructuralandstratigraphicrelations. Using a best-fit planar-tilt model and bootstrap analysis of uncertainties, we estimate that the paleohorizontal plane has a strike of 43& 630.4&and dip of 7& 62.0&southeast (695% confidence). Our estimated paleohorizontal allows us to restore the paleomagnetic data of Beck et al. (1981) and to estimate the original paleolatitude of the Mount Stuart batholith. Beck et al. found that the southern part of the batholith yielded a number of sites with a well-defined high-coercivity remanence. The carrier of this remanence was not resolved, but the following four lines of evidence stronglysuggestthatthepublisheddirectionswereacquiredshortly after emplacement of the batholith. (1) The ‘‘stable’’ sites all came from the shallowest and most rapidly cooled portions of the batholith as indicated by our AH results and concordant K/Ar ages for hornblende/biotite pairs. (2) The high coercivity component was always normal in polarity, which is consistent with emplacement of the Mount Stuart batholith at the end of the Cretaceous long normal. (3) Sites from the batholith and the contact aureole gave similar directions. (4) The directions show no indication of tilt-related smearing. After restoration, the paleomagnetic data indicate 42& 6 11& clockwise rotation and 3100 6 600 km of northward offset

The effects of temperature and fO2on the Al-in-hornblende barometer

Other| June 01, 1995 The effects of temperature and fO2 on the Al-in-hornblende barometer J. Lawson Anderson; J. Lawson Anderson University of Southern California, Department of Earth Sciences, Los Angeles, CA, United States Search for other works by this author on: GSW Google Scholar Diane R. Smith Diane R. Smith Trinity University, United States Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (5-6): 549–559. https://doi.org/10.2138/am-1995-5-614 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation J. Lawson Anderson, Diane R. Smith; The effects of temperature and fO2 on the Al-in-hornblende barometer. American Mineralogist 1995;; 80 (5-6): 549–559. doi: https://doi.org/10.2138/am-1995-5-614 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract The Al-in-homblende barometer potentially offers a basis for estimating crystallization pressure for granitic batholiths. However, owing to the simplicity of its formulation, misuse of the barometer can occur. Many granitic intrusions are emplaced at conditions inconsistent with those of the existing experimental calibrations, including fO2 < NNO and/or variable to high temperature. The barometer is sensitive to variations in both fO2 and temperature: low fO2 can cause calculated pressures to be high by a factor of two or more, and the effect of temperature is up to 2 kbar per 100 °C, depending on total Al abundance.Batholiths emplaced at elevated temperature and portions of plutons that crystallized below H2O saturation yield artificially high pressures relative to near-solidus experiments conducted at H2O saturation. Al-in-homblende pressures within the tonalitic Mount Stuart batholith of Washington, for example, define a 1–4 kbar domal structure that is exaggerated by emplacement crystallization temperatures that range from 620 to 760 °C. Pressure correlates with temperature (r2 = 0.86), indicating that temperature-sensitive edenite exchange has greatly influenced observed pressure variations. Data for other plutons also exhibit a marked positive correlation between temperature and hornblende pressure. If corrections are not made for temperature, such large apparent pressure variations can lead to overly high estimates of pluton thickness and tilt in addition to invalid tectonic interpretations.Using experimental data at ~675 °C (Schmidt, 1992) and at ~760 °C (Johnson and Rutherford, 1989), a revised expression for the barometer incorporating the effect of temperature is P (±0.6 kbar) = 4.76Al − 3.01 − {[T(°C) − 675]/85} × {0.530Al + 0.005294[T (°C) − 675]}. For a pluton emplaced at 100 °C above wet-solidus temperatures, this reformulation of the barometer lowers derived pressures by 1.3 to >2 kbar at typical crustal pressures. For the Mount Stuart batholith, consideration of temperature yields revised pressures that are in agreement with pressures obtained from wall rocks and eliminates much of the apparent domal structure.Low-/fO2 granites have amphibole Fe/(Fe + Mg) ratios that exceed the typical 0.40−0.65 range used in most experimental and empirical calibrations. Examples from anorogenic granitic batholiths of mid-Proterozoic age yield pressures that are too high by a factor of two to three in comparison with pressures obtained from adjacent metamorphic assemblages. Hornblende in these granites not only has high Fe/(Fe + Mg) but also low ratios of Fe3+ to Fe2+. The anomalously high Al in Fe-rich, Fe3+-poor hornblende is inferred to be the result of increased [6]Al occupancy of the M2 site not buffered by the Mg and Fe3+ abundances typical of amphiboles in calc-alkaline and other high-fO2 plutonic rocks. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The transition from magmatic to high-temperature solid-state deformation: implications from the Mount Stuart batholith, Washington

Criteria for syntectonic emplacement of plutons are commonly ambiguous. The strongest single criterion is probably the preservation of a continuous transition from submagmatic to high-temperature solidstate deformation. This transition is not commonly documented. An exception is the Mount Stuart batholith, which displays a variety of syntectonic structures within its sheared northeastern margin, including the following: (1) S- C fabrics that range from submagmatic to high-temperature solid-state;(2) solid-state foliation and lineation defined by amphibolite-facies assemblages that overprint and are parallel to magmatic foliation and lineation; (3) folds of magmatic and solid-state foliation that have hinge lines parallel to magmatic and solid-state lineation and to equivalent structures in wall rocks; (4) dikes that were folded and boudinaged over a wide range of rheological states with boudin necks locally filled by tonalite; (5) pegmatite dikes and mineralized joints that are sub-perpendicular to magmatic and solid-state lineation; and (6) submagmatic and high-temperature solidstate ductile shear zones. We suggest that many plutons emplaced during regional deformation do not preserve evidence for syntectonic deformation because of the transitory nature of the submagmatic state and the obscuring effects of postemplacement deformation. Syntectonic features are most likely preserved in plutons cooled at slow to moderate rates, or in plutons deformed at high strain rates during emplacement. The optimum conditions for preservation may occur in plutons emplaced along fault zones in the mid-crust, such as the Mount Stuart batholith, and in intrusions at deeper levels that were rapidly exhumed and/or intruded during the waning stages of regional deformation.

Tilt and northward offset of Cordilleran batholiths resolved using igneous barometry

CONSIDERABLE controversy surrounds the suggestion, based on palaeomagnetic evidence, that large segments of the North American Cordillera travelled long distances parallel to the coast during the latest Cretaceous and early Tertiary periods, well after the amalgamation of exotic terranes. Discordant palaeomagnetic data from mid-Cretaceous plutonic rocks of the Peninsular Ranges batholith of coastal southern California and Baja and the Mount Stuart batholith of the Cascade Range in Washington play a pivotal role in the controversy. The discordant data were originally interpreted to reflect northward transport of ≥1,000 km relative to cratonal North America, after the batholiths cooled through their magnetic blocking temperatures1–5. More recently it has been argued that the discordances arise from local tilting of batholiths, rather than northward offset6,7. Here we present and implement new methods based on hornblende barometry8 for determining the palaeohorizontal in granitic batholiths and correcting palaeomagnetic data for tilting. Our results indicate that the Peninsular Ranges and Mount Stuart batholiths have undergone northward offsets of ∼1,000 ±450 and ∼2,900±700 km, respectively, and also significant tilting.

Tectonic implications of syn- and post-emplacement deformation of the Mount Stuart batholith for mid-Cretaceous orogenesis in the North Cascades

The 93–96 Ma Mount Stuart batholith intruded across the boundary between the Northwest Cascades thrust system and the crystalline core of the North Cascades. Although previously considered posttectonic, the northeast margin of the Mount Stuart batholith and its wall rocks have been involved in syn- to post-emplacement, southwest-directed thrusting and folding, and west-northwest stretching. Contraction ended shortly after emplacement, as indicated by high-temperature recrystallization in thrust-related mylonites of the pluton and by geochronological data, whereas west-northwest stretching continued for an unknown period of time. This is the best documented mid-Cretaceous contractional belt in the main part of the crystalline core. The shortening direction and timing are identical to that of southwest-vergent thrusts in the offset continuation of the core in British Columbia. The contractional belt provides a link between thrusting in the Northwest Cascades thrust system and deformation in the crystalline core.

Revised metamorphic history for the Chiwaukum Schist, North Cascades, Washington

A revised metamorphic history for the Chiwaukum Schist, North Cascades, Washington, consisting of dynamic contact metamorphism of Buchan style associated with the Mount Stuart batholith, followed closely by regional metamorphism of Barrovian style, is similar to that for rocks adjacent to the Sloan Creek intrusion 30 km to the north, and on the opposite (west) side of the Straight Creek/Fraser River fault system, to the Settler schist in the vicinity of the Spuzzum and Scuzzy intrusions 200–270 km north in British Columbia. The Late Cretaceous regional increase in lithostatic pressure implied by this history, whether caused by tectonic or magmatic processes, may be the same as that responsible for the crystallization of magmatic epidote in certain other intrusives in the North Cascades such as the Tenpeak. More radiometric crystallization ages as opposed to cooling ages are needed to resolve this point.

The ophiolitic Ingalls Complex, north-central Cascade Mountains, Washington

The Ingalls Complex is the largest and most complete of several Middle to Late Jurassic ophiolites in northwestern Washington. It structurally overlies the Skagit metamorphic core of the North Cascades and is intruded by the Late Cretaceous Mount Stuart Batholith. The last truncates major structures of the ophiolite and divides it into two outcrop areas, a main area south of the batholith and a smaller area in a roof pendant. In the main area, the internal structure of the ophiolite is dominated by two wide fault zones characterized by steeply dipping serpentinite melange. Displacement probably was a combination of strike slip and dip slip, with the latter apparently related to diapiric rise of serpentinite. Deformation probably occurred in an oceanic transform fault zone. Ultramafic tectonites predominate in the main outcrop area, including from south to north: (1) harzburgites and dunites; (2) mylonitic iherzolites and hornblende peridotites; and (3) Iherzolites. Unit 2 represents a high-temperature shear zone that may record early movement in the deeper levels of the postulated oceanic fracture zone. Mafites generally occur as tectonic blocks; in the southern fault zone, they underwent ocean-floor-type metamorphism, but to the north, metagabbros experienced dynamothermal amphibolite-facies metamorphism prior to incorporation into the serpentinite melange. Gabbros in the southern fault zone locally intrude ultramafites and in places pass upward into a diabase dike complex. Gabbros and diabases intrude partly pillowed basaltic flows and pillow breccias, with interbeds of argillite, chert, and ophiolite-derived sedimentary breccia. Another sequence of basalt, argillite, chert, and minor limestone is penetratively deformed and may be exotic. In the pendant on the Mount Stuart Batholith, the north-directed Windy Pass thrust (WPT) places the Ingalls on the pelitic Chiwaukum Schist of the Skagit metamorphic core. The Ingalls is imbricated with amphibolite-facies metasedimentary and metaplutonic slices that may be a part of the Skagit core not exposed in the lower plate. Imbrication was synchronous with amphibolite-facies metamorphism of the ophiolite in the pendant, but the postmetamorphic sole thrust (WPT) truncates both the imbricate slices and fabric elements in the Chiwaukum Schist. Thrusting was mid-Cretaceous, apparently postdating high-angle faulting within the main portion of the Ingalls. The WPT marks a major Cretaceous boundary between an oceanic and a metamorphic terrane (Skagit core), and its upper plate represents the only rocks preserved above the Skagit core. The Ingalls is similar in age to ophiolites in California, Oregon, and British Columbia, as well as those in northwestern Washington. The California and Oregon ophiolites (Coast Range, Smartville, Josephine) apparently originated in small, short-lived marginal basins, whereas the Ingalls was deformed in a fracture zone in a wider, longer-lived marginal basin or large ocean.

Dynamothermal Contact Metamorphism Superposed on Regional Metamorphism in the Pelitic Rocks of the Chiwaukum Mountains Area, Washington Cascades

Research Article| July 01, 1980 Dynamothermal Contact Metamorphism Superposed on Regional Metamorphism in the Pelitic Rocks of the Chiwaukum Mountains Area, Washington Cascades Charles C. Plummer Charles C. Plummer 1Geology Department, California State University, Sacramento, California 95819 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1980) 91 (7_Part_II): 1627–1668. https://doi.org/10.1130/GSAB-P2-91-1627 Article history received: 17 Aug 1979 rev-recd: 21 Jan 1980 accepted: 05 Feb 1980 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Charles C. Plummer; Dynamothermal Contact Metamorphism Superposed on Regional Metamorphism in the Pelitic Rocks of the Chiwaukum Mountains Area, Washington Cascades. GSA Bulletin 1980;; 91 (7_Part_II): 1627–1668. doi: https://doi.org/10.1130/GSAB-P2-91-1627 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Chiwaukum Schist, in the Washington Cascade Mountains, has undergone two distinct episodes of metamorphism: (1) Regional Barrovian-type metamorphism and (2) subsequent “contact” metamorphism, associated with the Late Cretaceous intrusion of the Mount Stuart batholith, comparable to part of the Buchan-type progression.The later metamorphic event is unusual because the effects are predominantly dynamothermal, although in contact with and genetically related to plutonic intrusion. Some indications that penetrative deformation accompanied recrystallization during “contact” metamorphism are (1) preferred orientation of minerals such as cordierite, andalusite and sillimanite; (2) alignment of mafic minerals within granitic rocks near contacts; (3) granitic contacts being generally parallel to the predominant s-plane of the schist; (4) biotite within the schists showing more pronounced alignment the closer it is to granitic contacts; and (5) minerals in the schists near contacts commonly being microboudinaged and showing evidence for rotation during and subsequent to crystallization. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Dynamothermal contact metamorphism superposed on regional metamorphism in the pelitic rocks of the Chiwaukum Mountains area, Washington Cascades: Summary

The Chiwaukum Schist, in the Washington Cascade Mountains, has undergone two distinct episodes of metamorphism: (1) Regional Barrovian-type metamorphism and (2) subsequent “contact” metamorphism, associated with the Late Cretaceous intrusion of the Mount Stuart batholith, comparable to part of the Buchan-type progression. The later metamorphic event is unusual because the effects are predominantly dynamothermal, although in contact with and genetically related to plutonic intrusion. Some indications that penetrative deformation accompanied recrystallization during “contact” metamorphism are (1) preferred orientation of minerals such as cordierite, andalusite and sillimanite; (2) alignment of mafic minerals within granitic rocks near contacts; (3) granitic contacts being generally parallel to the predominant s -plane of the schist; (4) biotite within the schists showing more pronounced alignment the closer it is to granitic contacts; and (5) minerals in the schists near contacts commonly being microboudinaged and showing evidence for rotation during and subsequent to crystallization.