Geology of the southern part of the Chelan Block, Cascades Crystalline Core, WA
Matt Gatewood and Harold Stowell, Univ. of Alabama
GEO490/590 Sp 2010 Field Trip Day 6-7
- Contrast lithologic (compositional) variations in the Swakane terrane to evaluate potential protoliths for the Swakane Gneiss.
- Observe compositional variations, metamorphic textures/mineralogy, and deformation textures to provide a context for interpreting zircon and garnet ages.
- Compare leucogranite and pegmatite volumes in Swakane Gneiss outcrops from different structural levels within the Swakane terrane.
- Examine the Swakane/Napeequa contact (Dinkelman detachment).
- Evaluate contact relationships along the contact between the Entiat pluton and the Napeequa complex and the implications of c.a. 73 Ma heating during intusion of the Entiat pluton.
- Present the results of new and previously published geochronology and P-T data on the Swakane terrane.
- Discuss the differences in P-T-t histories between the Swakane/Napeequa and the adjacent Nason terrane (large pressure increases vs. moderate pressure increases, respectively).
Field Trip Stops: Day 6-7 (UTM zone 10T; NAD 27)
- Stop 6-1: Swakane/Napeequa on U.S.2 W. of Waterville (UTM 711977E 5279475N)
- Stop 6-2: Swakane metapelite on U.S.2/U.S.97 (UTM 708483E 5276059N)
- Stop 6-3: Swakane protolith on U.S.2/U.S.97 (UTM 708124E 5274479N)
- Stop 6-4: Swakane metamorphism at Rocky Reach Dam (UTM 703253E 5268025N)
- Stop 6-5: Swakane Gneiss in Swakane Canyon (UTM 694617E 5274964N)
- Stop 6-6: Deepest structural levels of the Swakane (UTM 703415E 5269305N)
- Stop 6-7: Swakane terrane at Vista Viewpoint (U.S.97a) (UTM 704199E 5269620N)
- Stop 6-8: Compositional variation in Swakane (U.S.97a) (UTM 707393E 5277819N)
- Stop 6-9: Uppermost Swakane Gneiss (U.S.97a) (UTM 707417E 5279696N)
- Stop 6-10: Entiat Pluton on U.S.97a (UTM 708330E 5282235N)
- Stop 6-11: Napeequa Complex on WA Hwy 19 (UTM 702057E 5283718N)
- Stop 7-1: Swakane/Napeequa in Roaring Caynon (UTM 697805E 5284228N)
The North Cascades mountains expose part of a Cretaceous-Paleogene magmatic arc and orogenic belt. Faults bound the eastern and western margins of this former arc and separate it from the Intermontane superterrane to the east and the Insular superterrane to the west. It is made up of a sequence of oceanic, island arc, and clastic-dominated terranes, that were mostly amalgamated before mid-Cretaceous orogenesis and plutonism (Tabor et al. 1989). Today, the lowermost portion of the arc exists as paired metamorphic belts that have been intruded by Cretaceous-Paleogene plutons (Fig. 1). These amphibolite-facies metamorphic rocks and their intrusives comprise the southern part of the Cordilleran Coast Plutonic Complex, which is continuous over 1500 km from northern Washington to the Yukon territory in Alaska. Exhumation of the North Cascades occurred during Eocene transtention (Johnson, 1985). This event produced large non-marine clastic basins and large brittle faults. The Cascades Core is divided into the Wenatchee and Chelan blocks by the post-metamorphic, high-angle Entiat fault (Fig. 1). Miller et al. (2000) used metamorphic pressure estimates from the Cascades Core to reconstruct a crustal section consisting of (from structurally lowest to highest): (1) Swakane Gneiss and overlying Napeequa complex (Chelan Mountains terrane), (2) Nason terrane (Chiwaukum Schist and Nason Ridge Migmatitic Gneiss), and (3) Ingalls Ophiolite Complex. The Swakane terrane/Swakane Gneiss is the lowermost exposed structural unit in the Chelan block of the Cascades crystalline core (Fig. 1). It is highly deformed, homogeneous, qtz+bt-pl±amp±ms±grt±ky±st gneiss with subordinate garnet amphibolite, calc-silicate, and ultramafics. Although the Swakane terrane contains no plutonic bodies of appreciable size, it is variably intruded by thin (< 10 m) leucogranite sheets and pegmatites. The Dinkelman detachment juxtaposes the Swakane terrane with the overlying Napeequa Complex (Tabor et al. 1989; Alsleben, 2000; Miller et al. 2000). The Entiat Fault separates the northwestern Swakane Gneiss bodies in the Wenatchee block from the southeastern body in the Chelan block. Because of accessibility problems (weather/terrain) and time constraints during our field trip, we will focus solely on Swakane rocks in the Chelan block.
Purpose of Swakane Field Trip 2010
The age and origin of the Swakane Gneiss are controversial subjects. U-Pb zircon ages have yielded a range of ages from 1610 Ma to 73 Ma (e.g. Mattinson, 1972; Matzel et al. 2002; 2004). This large range of zircon ages has led to debate over a silicic-volcanic or a sedimentary protolith for the Swakane Gneiss. Although the “remarkable homogeneity” of the Swakane Gneiss has been used to suggest a volcanic protolith (Mattinson, 1972; Cater, 1982), Miller et al. (2000) noted that the diversity of zircon ages would require inheritance from a “highly diverse crustal column”. Recent ID-TIMS U-Pb zircon ages suggest that Swakane lithologies were deposited as sediment as recently as 73 Ma (Matzel et al. 2004). Peak metamorphic conditions of 640-750°C; 9-12 kbar (Valley et al. 2003) must have occurred =5 m.y. after deposition, because post metamorphic leucogranite dikes and pegmatites have ages of ca. 68 Ma (Matzel et al. 2002; 2004). These detrital (zircon) and metamorphic (garnet and leucogranite) ages require burial to ~35 km and heating to partial melt conditions in <<5 m.y. after deposition. Zircon age constraints for the timing of deposition and peak metamorphism/partial melting translate into burial rates of ~7 mm/yr after deposition, which would likely result from loading during thrust sheet emplacement. However, heat conduction models suggest that an additional heat source, such as intruding magma, would be required to achieve peak temperatures of 640-750°C and partial melt conditions at a depth of 35 km in only 5 m.y. The absence of any significantly large plutonic bodies intruding the Swakane seems to negate this possibility, although magmatic underplating could have provided a less-obvious heat source. Additionally, this model requires that the Swakane Gneiss be loaded separately and at different times from the adjacent Nason terrane, which experienced peak metamorphism between 86-90 Ma. An alternative explanation of 73 Ma zircon ages is that peak metamorphism occurred before 73 Ma and these ages are not detrital, but metamorphic. We are testing this hypothesis by dating metamorphism using Sm-Nd isochrons to date garnet growth, and laser ablation inductively coupled plasma mass spectrometry to date chaotically zoned metamorphic zircon rims from the Swakane Gneiss. In-situ MC-LA-ICPMS U-Pb analyses of Swakane Gneiss zircon rims yield ages ranging from 65-94 Ma with distinct subpopulations at ~72, ~81 Ma, and ~91 Ma (Fig. 2). The ca. 72 Ma metamorphic rim ages are statistically distinguishable from older 81-94 Ma ages. The ca. 81 Ma and ca. 91 Ma age populations likely represent pulses of zircon growth during a 10-15 m.y. metamorphic event, because zircon rim ages span this age range (i.e. mixed ages between 80-94 Ma). Grains contain distinct cores with ages ranging from 2200 to 120 Ma [similar to Chiwaukum Schist DZ ages; N. Brown, personal com]. High U/Th ratios (>10) for 68-94 Ma chaotically zoned zircon rims suggest that they are of metamorphic origin (Fig. 2c). Furthermore, we have determined garnet-rock isochron ages of 71.3±2.8 Ma and 73.5±1.2 Ma for single-stage garnet from the Chelan block Swakane Gneiss (Fig. 3). While we haven’t dated garnet growth during the earlier (80-93 Ma) metamorphic event recorded in the zircon rims, we have identified several coarse-grained garnet amphibolite samples that have major element zoning profiles suggestive of polyphase growth. For future work, these samples may be appropriate for microsampling compositionally specific domains of garnet for geochronology. Using the spatial resolution afforded by the MC-LA-ICPMS technique, we show that zircons from the Swakane Gneiss are complex, with multiple metamorphic overgrowths of 68-73 Ma, and 80-94 Ma. The younger ca. 72 Ma zircon ages are indistinguishable from garnet Sm-Nd isochrons, suggesting that the detrital interpretation for ~73 Ma zircons is unlikely. Correlation of the =72 Ma metamorphic ages in the Swakane Gneiss with metamorphic ages in adjacent terranes indicates a shared tectonic history and allows for similar depositional histories. To augment our geochronologic dataset and provide a context for interpreting the age data, we are using metamorphic textures, mineral compositions, and pseudosections, to construct P-T paths for Swakane rocks. Contrasting the P-T-t paths for the Swakane with adjacent terranes will help elucidate the relationship of the Swakane terrane to the remainder of the North Cascades Crystalline Core.
Road Log (Fig. 4) Day 1: We will leave Daroga State Park (just north of Orondo, WA) and drive south on U.S. 97 towards Wenatchee (along the east bank of the Columbia River) to see pelitic schist and gneiss in the easternmost exposed rocks of the Swakane terrane. We will cross the Columbia River in Wenatchee and drive north along U.S. 97/alt, where we will stop frequently to examine Swakane terrane rocks from different structural levels. Just south of Entiat, WA, we will turn left and drive west along Chelan County Hwy 19, which follows the Entiat River valley. We will stop along this road to see Napeequa terrane rocks and the uppermost Swakane lithologies. We will camp at Pine Flat Campground near Ardenvoir, WA.
Day 2: Return to Chelan County Hwy 19 and turn left (SE). Turn right Roaring Creek Rd. at the Entiat Fish Hatchery. Continue ~1.8 miles past where pavement ends through the Forest Service gate and park in large flat area to left. We will spend 1/2 day here looking at the Dinkelman detachment. Afterwards, return to Wenatchee and drive to Leavenworth via U.S. 2.
Swakane Field Trip Stops
All Universal Transverse Mercator (UTM) coordinates are Zone 10, N.A.D. 27. Mineral abbreviations are from Kretz (1983).
Stop 6-1 (Optional) – Swakane/Napeequa contact.
UTM 711977E 5279475N – U.S. 2 east of Orondo, WA
Steeply dipping outcrops along U.S. 2 here expose part of the uppermost Swakane terrane and lowermost Napeequa Complex, which is intruded by the Entiat pluton. The Dinkelman detachment juxtaposes the more homogeneous Swakane gneiss against the compositionally layered Napeequa units here. Note the presence of ductile fabrics that have been overprinted by brittle ones in the Swakane and Napeequa units here. Early ductile fabrics are likely related to SW-directed thrusting of the Napeequa over the Swakane. Late ductile and brittle structures are extensional and likely related to reactivation of the Dinkelman during top-to-N shear and exhumation of the Swakane terrane. Tabor et al. (1987a) reported K-Ar ages of 48.4±2.2 Ma (hornblende) and 47.8±1.9 Ma (biotite) for two Swakane Gneiss samples from here. These cooling ages are significantly younger than a K-Ar biotite age of 50.8 Ma and hornblende ages of 64-71 Ma from the overlying Napeequa complex, which may indicate large displacements and excision of the Swakane along the Dinkelman detachment.
Stop 6-2: 08NC59 – Swakane Gneiss metapelite and crosscutting relationships.
UTM 708483E 5276059N – East side of Columbia River (along U.S. 2/U.S. 97) The rocks here are atypical of the homogeneous qtz-fsp-bt gneiss that constitutes most of the Swakane terrane. This outcrop contains pelitic schist intruded by thin (~15 cm) leucogranite and pegmatite sheets. Metamorphosed pelitic schist from this outcrop contains the peak assemblage: qtz+pl+bt+rt+ms+grt+ky (Fig. 5). Peak metamorphic conditions for garnet rims and matrix bt, ky, and pl from this outcrop are 725°C, 11.5 kbar (Ave PT). Phase diagram modeling of zoned garnet core composition suggests that initial garnet growth occurred at ~560°C, 5 kbar (Fig. 5). These data indicate significant P increases during metamorphism of the Swakane. This outcrop contains both deformed and undeformed (crosscutting) leucogranites and pegmatite sheets that contain qtz+pl+ms±grt±czo. Garnets are euhedral, small (0.2 mm), and inclusion-free. A deformed (folded and weakly foliated) leucogranite (08NC59c) from here produced complexly zoned zircon that yield a U-Pb crystallization age of c.a. 68 Ma. Leucogranite zircons contain inherited cores that range in age from 125-1445 Ma. Leucogranite and pegmatite compositions (ms and garnet bearing) and the presence of inherited (c.a. 1445 Ma) zircon cores suggest that these melts were derived locally from the Swakane Gneiss.
Stop 6-3: 08NC63 (Optional) – Compositional variation of the Swakane Gneiss.
UTM: 708124E 5274479N – East side of Columbia River (along U.S. 2/U.S. 97) These outcrops provide an opportunity for us to examine lithologic variation in the Swakane Gneiss and to consider its protolith. Although most would accept a sedimentary protolith for the calc silicate and metapelite, the source rock for the dominant qtz-pl-bt gneiss is disputed. Waters (1932) originally concluded that the Swakane was derived from arkosic sediments, while Mattinson (1972) and Sawko (1994) favored a felsic volcanic protolith because of the thickness and homogeneity of the unit. We will discuss the protolith of the Swakane in the context of its composition, zircon morphology and ages, and Proterozoic Sm-Nd model ages of 1.18 and 1.27 Ga (Rasbury and Walker, 1992). Several distinct lithologies are present here, including ms-bt-grt-ky schist (metapelite), qtz-pl-bt gneiss (metagraywacke), and ep-qtz-pl-bt gneiss (calc-silicate). Contacts between different rock types are sharp and parallel to compositional layering and metamorphic foliation, which trend 290°/25 NE.
Stop 6-4: 07NC11 – Metamorphism of the Chelan block Swakane Gneiss.
UTM 703253E 5268025N – U.S. 97A across from Rocky Reach Dam The Swakane Gneiss here contains cm-scale compositional layering that is parallel to metamorphic foliation. The darker (mafic) layers are bt-rich/qtz-poor and contain the peak assemblage bt+pl+ms+chl±qtz with accessory ky, rt, ilm, and relict st. The lighter (felsic) layers contain the assemblage qtz+pl+bt+grt with accessory chl, rt, and ilm. Garnets from the qtz-rich layer are 3-8 mm, euhedral, and inclusion-poor, in contrast to garnet in the mafic layers, which are anhedral and inclusion-rich. Garnet from both layers preserve growth zoning profiles (Mn-rich cores) and sharp Ca increases near the rim, probably related to pressure changes during garnet growth (Fig. 6). Garnet rim compositions, paired with matrix biotite and plagioclase compositions yield Average PT results of 709°C, 11.9 kbar, indicating that these are some of the deepest exposed rocks in the North Cascades. The felsic layer yielded a 5-point garnet-rock isochron age of 73.5±1.2 Ma (MSWD=1.7; prob=0.16; Fig. 3b). This age is indistinguishable from a 71.3±2.8 Ma garnet-cpx-rock isochron for a garnet amphibolite near the Dinkelman detachment (Stop 7-12; Fig. 3a). Together with 73 Ma rims on detrital zircon, these garnet ages document a ca. 73 Ma metamorphism of the Swakane Gneiss in the Chelan Block. We will discuss the significance of this age and compare it to metamorphic ages for the adjacent Nason terrane (Wenatchee Block). If we walk ~80 meters to the north along the road, we can see an example of the broad open folds that overprint earlier isoclinal and recumbent folds. These open folds trend E-W and likely developed during top-to-N shear, synchronous with the development of extensional crenulation cleavage (shear bands), asymmetric boudinage, and subhorizontal shear surfaces (Miller et al., 2000). Late structures deform ca. 68 Ma pegmatite sheets that crosscut metamorphic foliation, which constrains the timing of top-to-N shear. We will discuss the tectonic significance of these late structures and fabrics and their relationship to the exhumation of the Swakane terrane along the reactivated Dinkelman detachment. Mafic layers here contain the assemblage grt+pl+amp+bt+qtz±spn+rt±ilm±czo. Garnet major element zoning profiles show sharp discontinuities, suggestive of polyphase growth (Fig. 7). Significantly higher Ca in the garnet rims suggests that secondary garnet growth is higher P than the first. Rutile overgrowths on matrix ilmenite grains support increasing P during progressive metamorphism. Average P-T results for high-Ca garnet rims and a matrix plagioclase, biotite, amphibole compositions are 766±63°C, 11.5±1.1 kbar (8 reactions). These garnets may be suitable for future geochronology work and potentially record the earlier ca. 80-93 Ma episode of metamorphism recorded in zircon rims.
Stop 6-5 (Optional): 07NC28 – Westernmost Swakane Gneiss near Entiat fault.
UTM 694617E 5274964N – Swakane Caynon Rocks from this outcrop are more typical of the Swakane qtz-fsp-bt gneiss than those we’ve seen so far. We are on the west limb of a regional antiform and at slightly higher structural levels than at Stop 6-4. The Entiat fault truncates the west flank of this antiform just to the west of here. This outcrop contains fine-grained, homogeneous, qtz-rich gneiss with the assemblage qtz+pl+bt+czo+ox±grt. Samples from here display a strong fabric that trends 314/28 SW, but compositional layering is indistinct due to homogeneity and small grain size. I chose this outcrop to sample for zircon metamorphic rim geochronology because of the lack of intrusive material (leucogranite and/or pegmatite) here. We will discuss the zircon ages from this outcrop at Stop 6-7, where we can compare intrusive-rich with intrusive-poor outcrops.
Stop 6-6 (Optional): 07NC30 – Deepest exposed levels of the Swaknae terrane.
UTM 703415E 5269305N – Lincoln Rock/Swakane Caynon on U.S. 97A We are in the hinge zone of a regional, gently NW-SE plunging antiform here (Fig. 1). This structure controls the map pattern of the Swakane Gneiss in the Chelan block. Outcrops near the top of the cliff are compositionally similar to those at the last stop and contain the assemblage qtz+pl+bt+ox±mu±chl±czo±spn. Thermobarometry on samples from this part of the Chelan block have consistently given high P results (8-12 kbar; Valley et al., 2003; 07NC11&12, this guidebook). Outcrops of these high-P rocks (here and along the Columbia River) typically contain numerous outcrop-scale intrusive bodies. The outcrop near the top of the cliff was chosen for zircon metamorphic rim geochronology, because of its structural position within the Swakane Gneiss and the presence of local intrusives. Zircons from here have distinct rounded cores with blurred and convolute zoning that have been overgrown by thick (45-50 um) homogeneous metamorphic rims (Fig. 8). Rounded cores are Proterozoic in age and rims have ages of 93-70 Ma. This outcrop also produced three zircon grains with younger (~93 Ma) cores that have high U/Th ratios and sector zoning and are interpreted to be partially recrystallized (metamorphic) zircon grains (Fig. 8c).
Stop 6-7: 07NC22 – 73 Ma zircon and the tectonics of the Swakane terrane.
UTM 704199E 5269620N – Viewpoint overlook on U.S. 97A The spectacular (infamous?) outcrops here have been the subjects of several structural, petrologic, and geochronologic studies that have resulted in differing interpretations about the timing of deposition and subsequent metamorphism of the Swakane Gneiss (e.g. Mattinson (1972); Alsleben (2000); Boysun and Paterson (2002); Matzel et al. (2004); Gatewood and Stowell, (2008; 2009)). The Swakane Gneiss here contains the assemblage qtz+pl+bt+ox±mu±czo and is cut by numerous leucogranite and pegmatite sheets and quartz veins. Composition of intrusives is similar to those at Stop 2 (ms and grt bearing). As we will see at the next two stops, the density of intrusive material decreases at higher structural levels. Outcrops on both sides of the road contain strong transposition fabrics (foliation and lineation) and several generations of folds (early isoclinal and recumbent folds overprinted by open folds, extensional crenulation cleavages, and asymmetric boudinage). This is the outcrop that produced zircon grains with ID-TIMS ages of 72-73 Ma (Matzel et al., 2004). These eight zircon grains were used to infer that the Swakane protolith was deposited after 72-73 Ma and subsequently buried to 35 km by 68 Ma. A detrital interpretation of ~73 Ma zircon requires rapid burial rates of ~7 mm/yr over 5 m.yr., presumably by thrust loading. Heat conduction models indicate that it would be unlikely for the Swakane rocks to reach 650-750°C in < 5 m.yr. without an additional heat source, although the ~73 Ma Entiat pluton may have provided one. A new sample of qtz-pl-bt gneiss from here (07NC22) produced zircon with LA-ICPMS spot ages of 68-1675 Ma. Most grains contain distinct rounded cores that are Proterozoic age and have thick (20-50 um) homogeneous rims that are 68-94 Ma (Fig. 9). This sample also contained zircon with 125-150 Ma oscillatory-zoned cores and a few grains with sector-zoned or homogeneous cores younger than 95 Ma. A single zircon crystal from this sample produced a 206Pb/238U age of 71.7±2.1 Ma for the zircon core, which is identical to the rim age. However, this zircon contains no distinct core, displays patchy zoning with homogeneous zones, and has high U/Th (Fig. 9c). Based on CL zoning patterns, single-grain U-Pb age distributions, high U/Th ratios, and 73 Ma garnet Sm-Nd ages, I argue that ca. 73 Ma zircon ages are metamorphic in origin and do not represent an upper bound for the depositional age of the Swakane Gneiss.
It is important to note that this is the only outcrop of Swakane Gneiss in any study to produce zircon with cores as young as 73 Ma. Considering the high density of intrusive material here and the penetrative deformation fabric of the rocks, it is plausible that 72-73 Ma oscillatory-zoned zircon (from Matzel et al. 2004) grew in leucogranite melts or pegmatite fluids ~73 Ma and was subsequently incorporated into the gneiss during high-T deformation. Regardless of whether ~73 Ma zircon in this outcrop grew from metamorphic fluids or in a intrusive body and was subsequently incorporated into the gneiss during ductile shearing, 72-73 Ma metamorphic ages rule out the possibility that the Swakane protolith was being deposited post 73 Ma. This interpretation does not require rapid loading (>5mm/yr) of the Swakane terrane from 72-68 Ma. Furthermore, 81-93 Ma metamorphic zircon rims record an early (pre 73 Ma) metamorphic episode, similar to metamorphic ages in the adjacent Nason terrane, indicating a shared tectonic history and allowing for similar depositional histories.
Stop 6-8: 07NC41 – Compositional variation and Swakane protolith.
UTM 707393E 5277819N – Turnout/small pond on left side of U.S.97 We are at shallower structural levels than at the previous stop. Here, we can explore the lithologic variation of the Swakane Gneiss. This outcrop contains garnet-staurolite schist and garnet amphibolite layers/lenses within the Swakane Gneiss. Similar to stops 6-2 and 6-3, the compositional heterogeneity here provides an opportunity to discuss potential protoliths for the Swakane. Note the well-developed folds in the schist and amphibolite layers here. Bt schist layers here contain relict staurolite that is partially replaced by kyanite, suggesting increasing P during metamorphism. Using conventional thermobarometry, and pairing garnet and plagioclase cores of zoned grains, Sawko (1994) produced P-estimates of 8 kbar. Also, his garnet and plagioclase rim compositions produced P-estimates of 10-12 kbar, suggesting significant P-increases during loading. These results are similar to those of Brown and Walker (1993) and Whitney et al. (1999), and contrast with those of Stowell et al. (2007), which require only moderate (1-2 kbar) P-increases during metamorphism of the adjacent Nason terrane.
Stop 6-9: 07NC35, 41 (optional) – Uppermost Swakane Gneiss.
UTM 707417E 5279696N – Large outcrop on the right side of US97A We are at the highest structural levels than we have been at since Stop 6-1. The trace of the Dinkelman detechment is just to the north of here, but is not exposed along the road. We will, however see it exposed tomorrow morning in Roaring Canyon, when we will be at a structural level comparable to here. This outcrop consists of strongly foliated, homogeneous, qtz-fsp-bt gneiss that lacks the abundant leucogranite and pegmatite intrusive sheets present at deeper structural levels. Note abundant intermediate to mafic dikes, which postdate ductile deformation and are probably Eocene in age. Valley et al. (2003) reported P-T estimates of 750°C, 11 kbar for a grt-ky gneiss sample from near here.
Stop 6-10: 07NC8 – Southwestern margin of the Entiat pluton.
UTM 708330E 5282235N – U.S. 97A, just south of the town of Entiat. The following stop description is modified from Miller et al. (2000), their Stop 2-1. The 72-73 Ma Entiat pluton consists of coarse-grained bt-hbl tonalite here. The map pattern shows the Entiat and Seven Fingered Jack intrusive suite as a highly elongate intrusion, continuous for > 65 km along strike (Fig. 1). Matzel (2004) concluded that the Entiat pluton is compositionally heterogeneous and does not represent crystallization of a single magma type. Based on the presence of magmatic epidote and Al-in-hornblende barometry, Dawes (1993) concluded that the Entiat pluton crystallized at 6-7 kbar. Although the contact between the Entiat pluton and the adjacent Napeequa complex shows evidence for high strains, Alsleben (2000) and Paterson and Miller (1998) interpret the contact to be intrusive based on the presence of Napeequa rafts and numerous Entiat-like sheets in the Napeequa near the contact. Here, at its southeastern end, the Entiat pluton contains numerous microgranite enclaves, schlieren layers, and a strong magmatic foliation. This is a good place to consider the Entiat pluton as a ca. 73 Ma heat source for metamorphism in the Chelan block.
Stop 6-11: 07NC34, 08NC66 – Napeequa Complex.
UTM 702057E 5283718N – Large outcrop on Hwy 19 This is our first chance to get a look at the Napeequa Complex (Mad River terrane of Tabor et al., 1989) that is in fault contact with the Swakane terrane. Based on its rock types and compositional heterogeneity, Tabor et al. (1989) interpreted the Napeequa Complex as an oceanic assemblage that was deformed in an accretionary wedge. Valley et al. (2003) reported P-T estimates of 685°C, 10 kbar for peak metamorphism of a nearby garnet amphibolite and interpreted clockwise P-T paths, similar to P-T results for the underlying Swakane. Here, the Napeequa consists of quartzite (metachert), siliceous schist, and hbl-bt gneiss, with lesser amounts of grt amphibolite and marble. The rocks are intruded by large (on the hill to NE) and small (in the outcrop) orthogneiss bodies, which are ca. 68 Ma (H. Hurlow).
Stop 7-1: 07NC31, 32 – Dinkelman detachment, uppermost Swakane, lower Napeequa. UTM 697805E 5284228N – Roaring Creek and FS Road 5101 The Dinkelman detachment is well exposed here on the cliffs on both sides of Roaring Creek (Fig. 10). These are interesting outcrops and several hours are needed to explore the area. The Swakane Gneiss here is more compositionally variable (grt-bt gneiss layers and grt amphibolite layers) and intruded by numerous pegmatite and leucogranite sheets. A short hike up the west side of the drainage (SE of Roaring Creek) will allow a look at the coarsening of metamorphic mineralogy along the Dinkelman detachment and the mylonites in the fault zone. Garnets here are =1 cm and kyanite crystals are up to 3 cm in length. Valley et al (2003) provided P-T estimates of 650-700°C and 10-12 kbar for peak metamorphism and T estimates of ~550°C for retrograded (chl-ms bearing) samples from here. We are now just beneath the Dinkelman detachment at structural levels similar to those at stop 6-9. Metamorphic foliation is continuous across the Swakane/Napeequa contact here, although the Swakane doesn’t contain the pervasive folding and lineations present in the Napeequa. Several low-T discreet shear zones that contain chl and ms and give top-to-N shear sense occur in these outcrops. This is a good time to discuss the tectonic significance of the Dinkelman detachment and reactivation of an early SW-directed thrust as a normal-sense shear zone during top-to-N shear and exhumation of the Swakane Gneiss (lower plate).
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