2. Department of Earth and Environmental Sciences, California State University, Fresno, California 93740, USA;
3. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;
4. Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China
Ophiolites are defined as distinctive assemblages of mafic to ultramafic rocks that represent pieces of oceanic lithosphere exposed on land (Moores, 1982; Coleman, 1971). The fragments of old oceanic crust and lithospheric mantle have been tectonically emplaced onto continental margins, island arcs or within accretionary complexes (Robinson et al., 2008; Moores, 1982; Dewey, 1976). A more restricted definition of ophiolites is that on-land oceanic lithospheric fragments were not partly subducted (Moores, 1982). For ophiolites of this restricted definition, their basal tectonic contact is a paleosubduction zone (Moores, 1982, 1970; Dewey and Bird, 1970), and more specifically, a subduction initiation horizon (Wakabayashi and Dilek, 2003). Accordingly, such "upper plate" ophiolites, which include the best-known ophiolites of the world, represent particularly important markers in orogenic belts, and the details of their formation and emplacement provide valuable insight into orogenic processes (Peng et al., 2016; Zhou et al., 2016; Dewey and Bird, 1970; Moores, 1970).
Metamorphic soles, found structurally beneath many ophiolites, are highly strained, high-grade metamorphic thin units (generally ≤500 m thick) (Spray, 1984; Williams and Smyth, 1973). They comprise mainly metabasites with subordinate meta-pelagic sediments and display inverted metamorphosed gradient (Jamieson, 1986; Spray, 1984) and anticlockwise pressure-temperature-time (P-T-t) paths (Guilmette et al., 2008; Dilek and Whitney, 1997; Wakabayashi, 1990). Some researchers proposed that they are formed as a result of subduction initiation beneath a hot, young oceanic lithosphere (Hacker, 1990; Jamieson, 1986; Spray, 1984; Williams and Smyth, 1973). Accordingly, metamorphic soles represent a key structural horizon within orogenic belts that records major processes of tectonic transition.
This paper provides a detailed report of weakly metamorphosed crustal gabbroic rocks in the Feather River ultramafic belt (FRB). They were originally mapped as gabbro or metagabbro by Hietanen (1981). Hietanen's (1981) map shows other gabbro bodies along the FRB, but the gabbro bodies that have been reexamined to date by our research group have proven to be clinopyroxene-bearing amphibolites, with the apparent gabbroic texture formed entirely by high-grade metamorphism (Masutsubo and Wakabayashi, 2013).
We present field, petrographic, metamorphic, and geochemical data from an upper plate ophiolite remnant, and evaluate some of the unusual specific relationships between this upper plate ophiolite remnant and the amphibolitic rocks that may include metamorphic soles. These relationships bear on processes of oceanic crust formation, subduction initiation, emplacement of ophiolites, and differences in the nature of ophiolitic occurrences in different orogenic belts, such as the North American Cordillera.1 GEOLOGICAL SETTING
The ophiolitic rocks described in this paper are part of the 150-km-long Feather River ultramafic belt (FRB) of the northern Sierra Nevada, California (Figs. 1a, 1b), that represents a major ophiolitic suture within the greater North American Cordilleran Orogen (Saleeby et al., 1989; Ehrenberg, 1975; Williams and Smyth, 1973). The FRB is unusual among Cordilleran ophiolitic belts because of the rarity of crustal rocks, especially crustal rocks lacking significant metamorphism (upper plate ophiolite remnants). It consists of serpentinized peridotite, with volumes of amphibolite grade metabasites and much less metagabbro (Fig. 1c). The amphibolite facies metabasites comprise primarily metabasites with lesser amounts of metacherts and metaclastic rocks that record high-temperature (HT) and medium to high-pressure metamorphic conditions (Masutsubo and Wakabayashi, 2013; Smart and Wakabayashi, 2009). These high-grade metamorphic rocks have been suggested to have formed metamorphic soles (Smart and Wakabayashi, 2009; Ehrenberg, 1975).
The FRB is fault-bounded by Calaveras complex to the west and Shoo Fly complex to the east (Fig. 1b). The Calaveras complex is regarded as a subduction complex mainly consisted of phyllite-argillite and metachert with some volcanic units (Edelman et al., 1989; Sharp, 1988; Hietanen, 1981). Fossil and isotopic data shows that Calaveras complex is growing westward (Bateman et al., 1985). An upper bound on the age of accretionary process of Calaveras complex is provided by an intruded pluton, yielded a concordant U/Pb zircon age of 177 Ma (Sharp et al., 1988).
The Shoo Fly complex consists of imbricated, mostly metaclastic rocks, with subordinate metachert, and serpentinite (Hannah and Moores, 1986). These have been metamorphosed at lower greenschist grade (Hannah and Moores, 1986). The structurally highest subunit of the Shoo Fly complex is known as the Sierra City mélange, and this unit has a significant proportion of serpentinized ultramafic rock as well as demonstrable block-in-matrix character (Hannah and Moores, 1986). Geochronologic data is sparse but depositional ages of most of the Shoo Fly complex appear to be Ordovician to Devonian based on Silurian–Ordovician fossils, and the Late Devonian age of units that unconformable overlie the Shoo Fly or cross cut it (Hannah and Moores, 1986). Although it has been proposed as a subduction complex, on the basis of its assemblage of lithologies and internal structure, the marked difference between Sierra City mélange in lithologies and detrital zircon age populations, suggest that the former may have a different tectonic origin and affinity than the other components of the Shoo Fly complex (Gehrels et al., 2000; Harding et al., 2000).
From west to east, the Calaveras complex, FRB, and Shoo Fly complex represent progressively older units accreted to the western margin of North America. The primary (original) regional contact geometry between these units is difficult to ascertain, however, owing to significant deformation and out-of-sequence faulting that postdated the original juxtaposition of these units (Edelman and Sharp, 1989). The Calaveras complex appears to be structurally beneath the FRB based the east-dipping orientation of the contact between the two units and the west-vergent high- temperature fabrics in the metamorphic sole of the FRB in the northern Feather River region (Smart and Wakabayashi, 2009).
The contact geometry between the FRB and Shoo Fly is more difficult to interpret. Much of this contact in the northern Sierra Nevada dips at high angles as can be seen in the regional-scale contact geometry. In contrast, in the central Sierra Nevada, the Shoo Fly complex overlies the Calaveras complex along a low-angle fault that has a regional easterly dip. Along this low-angle contact are local bodies of ultramafic, mafic, and high-grade metamorphic rocks that appear to the southern continuation of the FRB (Jackson et al., 2011). This suggests that the FRB is structurally beneath the Shoo Fly complex, although much of the contact between the two units may be a later high-angle fault.
In the vicinity of the field area, the Shoo Fly complex flanks the gabbro and serpentinite on the west and east (Figs. 1, 2). Examination of the map pattern (Hietanen, 1981) suggests a regional-scale low-angle contact between Shoo Fly complex and FRB rocks, with some late faulting along NNW-striking reaches of the contacts.2 FIELD RELATIONSHIPS AND STRUCTURAL GEOLOGY
The upper crustal ophiolitic rocks (metagabbro shown in Fig. 1c) form a outcrops covering about 2 km2 along La Porte Road and between serpentinite of the FRB and metaclastic rocks of the Shoo Fly complex (Figs. 1b, 2). The serpentinite is a part of a regionally extensive continuous serpentinite belt making up one of the main exposures of the FRB (Fig. 1b). The three basement units are overlapped by Miocene andesitic volcanic rocks to the north (Fig. 2) (Hietanen, 1981).
Most of the serpentinite outcrops are foliated but some are massive (Fig. 3a). The foliation generally dips steeply eastward and locally displays southward dips (Fig. 2). Relict peridotite textures are preserved in some outcrops (Fig. 4a). The western contact of the serpentinite unit with metagabbro unit is easily discerned on the basis of the rock color difference and vegetation contrasts in aerial photos (such as Google Earth) as well as being easily traceable on the ground (Fig. 3e).
The upper crustal mafic rocks are composed primarily of hornblende gabbro (Fig. 3f) with some clinopyroxenite. Although this rock body covers about 2 km2, it extends an indeterminate distance further northward beneath the Miocene cover strata. Some outcrops of gabbro display a high-temperature subsolidus fabric (Fig. 4c), whereas much of the gabbro lacks a penetrative fabric. Some other outcrops of gabbro have pegmatitic selvages (Fig. 4d). Diabase dikes intrude the gabbro and are locally common, but sheeted dikes were not found. The gabbro with high T shear fabric includes some fine-grained selvages that are boudinaged dikes (Fig. 4c). This suggests dike intrusion prior to the high T deformation in the gabbro.
The western contact of the gabbro and serpentinite against Shoo Fly complex rocks truncates the gabbro serpentinite contact at a high angle (Fig. 2). Whereas foliation dips in the Shoo Fly dip steeply eastward, placing the Shoo Fly beneath the gabbro body, the general contact geometry is that of a gently folded high-angle contact (Fig. 3c). As noted earlier, regionally the Shoo Fly structurally overlies the FRB rather than underlying it. In the field area the Shoo Fly complex comprises primarily sandstone with minor siltstone and shale (Fig. 3d). These rocks have a pervasive foliation (Fig. 4b).3 PETROGRAPHY 3.1 Serpentinite
The serpentinized ultramafic rocks consist of serpentinite and talc schist with local preservation of relics of olivine and pyroxene (Fig. 5a). The serpentinite comprises mainly antigorite with some lizardite and chrysotile. The talc schist consists of talc, antigorite and usually chlorite.3.2 Shoo Fly Complex Sandstone
The Shoo Fly complex comprises primarily sandstone with minor siltstone and shale with a pervasive foliation (Fig. 4b). It is mainly quartz grains, slightly altered feldspar and various lithic grains as well as some recrystallization of quartz lithics as a result of deformation (Fig. 5c).3.3 Gabbro Unit
The hornblende gabbro consists of (altered) plagioclase, hornblende, and clinopyroxene. The primary amphibole is a brownish amphibole that appears magmatic in origin based on preservation of primary magmatic textures that include clinopyroxene and plagioclase (Fig. 5e). Later generations of green to pale green amphibole rim the primary amphibole and clinopyroxene (Fig. 5e). In some samples, brownish amphibole is deformed (Fig. 5d) and locally recrystallized as part of a high-temperature fabric that also affected the plagioclase and clinopyroxene. In others, there are quartz veins that may have originated as tension gashes that cross cut the early high temperature fabrics of clinopyroxene and amphibole (Fig. 5f). This quartz is ductily deformed itself and exhibits ribbon and subgrain textures. Quartz is also locally present segregations that are apparently associated with the metamorphism of the gabbro. Much of the plagioclase has been altered to albite and fine-grained mats of minerals that are difficult to distinguish without electron microprobe analyses (see mineral composition section). Chlorite is common as a replacement of amphibole and clinopyroxene. White mica ranges from 0.2 to 1 mm size, and appears as texturally late platy aggregates associated with alteration of plagioclase. Opaque minerals including titanite and ilmenite are rare and range up to 0.3 mm.
The pyroxenite is massive and composed of mainly clinopyroxene. The clinopyroxene reaches 0.5–1 mm size and displays a cumulus texture. Diabase dikes are considerably finer-grained than the gabbro and display a similar mineralogy (Fig. 5d).4 ANALYTICAL METHODS
Three representative thin sections of gabbro were prepared for mineral composition analysis. In addition, eleven samples were crushed and 200 μm mesh powders were prepared for major and trace elements analysis.4.1 Mineral Compositions
Mineral compositions of the selected gabbro were analyzed by using a JEOL 8100 microprobe analyzer in Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). Analytical conditions were 15 kV accelerating voltage, 20 nA beam current, a spot diameter of 1 μm and 20 s counting time. Representative compositions of clinopyroxene, amphibole are listed in Tables 1, 2.
Whole rock major and trace elements for the selected gabbros and dikes were determined at the National Research Center of Geoanalysis, Beijing. Major elements were analyzed on fused glass disks by X-ray fluorescence spectrometry (XRF) and trace elements were examined by inductively coupled plasma-mass spectrometry (ICPMS) following the techniques of Liang et al. (2000). Errors for major element concentrations are within 1% and for most trace elements (including REE) are within 10%. The analyzed geochemical data of dikes and gabbro are listed in Table S1.5 RESULTS 5.1 Mineral Chemistry of Metagabbro 5.1.1 Clinopyroxene
The clinopyroxene is usually rimmed by amphibole (Figs. 6a-6d) and anorthite under back scatter electron (BSE) image. Most of the clinopyroxene has the high content of MgO (12.95 wt.%–15.73 wt.%) and low content of CaO (20.77 wt.%–23.14 wt.%) (Table 1). The clinopyroxene compositions are diopside according to clinopyroxene classification of Morimoto (1988) (Fig. 7).5.1.2 Amphibole
Two general types of amphibole are recognized: an early igneous amphibole and later actinolitic amphibole that rims the igneous amphibole and clinopyroxene (Figs. 6e, 6f). The primary igneous amphiboles are magnesiohornblendes. We could not see a clear distinction in composition between the syntectonic brown amphibole associated with high-temperature fabrics and the igneous amphiboles. The early-formed amphiboles are replaced by and rimmed by less aluminous calcic amphiboles, the texturally latest of which are actinolite. The post-magmatic actinolitic amphibole generally displays progressively lower Al2O3 and TiO2 with successive textural generations (Table 2). Aluminum and titanium contents of amphibole are positively correlated (Fig. 8).5.1.3 Feldspars 5.1.4 Ti phase minerals
The Ti phase minerals are mainly titanite and ilmenite. They are presented as trace amounts in the gabbro. Both of them appear very pale in BSE images. Their mineralogical identity was confirmed by the nature of their energy dispersive spectra, but quantitative chemical analyses were not performed.5.2 Geochemistry of Dikes and Gabbro 5.2.1 Dikes
The major element compositions of the dikes are characterized highly variable SiO2 (40.46 wt.%–65.03 wt.%) and moderate to high Al2O3 (13.6 wt.%–16.8 wt.%) (Table S1). MgO contents range from 1.47 wt.% to 13.82 wt.%, with Mg#=58.64–74.16. K2O and TiO2 concentrations are comparatively low. The general compositions span a large range from intermediate to ultramafic composition.
The rare earth elements (REE) are strongly fractionated, as indicated by low heavy REE (HREE) abundances (Yb < 2.47 ppm) and high (La/Yb)N ratios (8.09–39.45) (Table S1). Primitive- mantle-normalized trace element patterns display a weak negative Eu anomaly, strong depletions of Nb, Ta and Ti, and slight depletion of Zr and Hf (Fig. 10a). Chondrite-normalized REE patterns show LREE enrichment compared to the HREE (Fig. 10b).5.2.2 Gabbro
The SiO2 content of the gabbro spans limited range of 43.54 wt.% to 46.89 wt.% and Al2O3 content ranges from 15.64 wt.% to 18.53 wt.%. K2O and Na2O contents are very low. Mg# ranges from 61.38 to 80.45. TiO2 content ranges from 0.29 wt.% to 1.53 wt.%. Primitive-mantle-normalized trace element patterns (Fig. 10c) show prominent positive Pb and Sr anomalies and negative Zr and Hf negative anomalies. In the chondrite-normalized REE patterns (Fig. 10d), are slightly flat with positive Eu anomalies.6 DISCUSSION 6.1 Discussion of Mineral Chemistry and Geochemical Results 6.1.1 Conditions of metamorphism
Electron microprobe analyses showed typical igneous compositions for the clinopyroxene and texturally early hornblende, consistent with igneous textural relationships recorded by petrographic and BSE imagery analysis. The textural compositional relationships of later amphibole are complex. Some early metamorphic amphibole is associated with high-temperature fabric and its composition is indistinguishable from the apparent igneous amphibole. The high-temperature fabric, also observed at outcrop scale (Fig. 4c) is defined by alignment of the long axes of syntectonic amphibole, bending of earlier-formed amphibole, and possible development of subgrain texture in the plagioclase (Fig. 5f), although the latter is speculative owing to the severe alteration of plagioclase. Later textural generations of amphibole show a range in Al contents and the more aluminous (and comparatively early) of them may reflect higher temperature conditions than that reflected in the pumpellyite-actinolite assemblages in the same rock and the adjacent units (units adjacent to the FRB).
The white mica may give additional insight into the metamorphic history of the gabbro. Its composition is nearly end member muscovite and differs significantly from the more phengitic white mica associated regional metamorphism of units in the northern Sierra Nevada (Smart and Wakabayashi, 2009).
The earliest stage of this metamorphism was associated with high-temperature shear zones probably associated with oceanic detachment faulting. This deformation and metamorphism appears to have been partly coeval with oceanic magmatism as shown by the boudinaged dike rocks (Fig. 4c) and the early-formed syntectonic amphibole (Fig. 5d) that is indistinguishable in composition from the igneous amphibole. The high-temperature fabric is seen in the gabbro, but not the dikes, and the localization of high-temperature shear zones in the lower plutonic sections of the oceanic crust is consistent with deformation associated at a spreading center. The high-temperature fabric is also cut by later zones of brittle-deformed quartz (Fig. 5f) that in turn predate some growth of late pumpellyite. The other late metamorphic minerals are actinolite, albite, clinozoisite, and chlorite. Such mineral assemblage is similar to the pumpellyite-actinolite to lower greenschist assemblages of the neighboring Shoo Fly and Calaveras complexes, but contrasts starkly with the amphibolite grade metamorphism found elsewhere in the FRB.
The white mica and texturally intermediate amphiboles (less aluminous than igneous amphibole, but more aluminous than latest stage actinolite) were probably associated with sea floor metamorphism that postdated the high-temperature detachment faulting but predated low-grade burial metamorphism of the gabbro.
Accordingly, the gabbro and dikes both contain low grade metamorphic minerals such as pumpellyite, actinolite and albite, which indicate that they bear a low grade metamorphism (burial metamorphism). This burial metamorphism overprints earlier sea floor metamorphism and associated fabrics. The low-grade burial metamorphic assemblages is similar to the assemblages in the adjacent Shoo Fly and Calaveras complexes as well as the regional metamorphic overprint recorded throughout the northern Sierra Nevada metamorphic rocks (Hacker, 1993). The gabbro represents low-grade crustal metamorphic rocks found in the FRB.6.1.2 Geochemistry
Because the dikes and gabbro are little metamorphosed their geochemistry can provide insight into the tectonic setting of the magmatism that formed these ophiolitic rocks. The high field strength elements (HFSEs), the rare earth elements (REEs) and some transitional elements (such as Ti, V) are thought to be generally immobile during hydrothermal ocean floor alteration and metamorphism (Dilek et al., 2011), although certain major (Si, Na, K, Ca) and trace (Cs, Rb, Ba, Sr) elements may be modified during hydrothermal seafloor alteration (Gillis, 1995).
The dikes and gabbro show Nb and Ta depletion in primitive-mantle normalized plots that is typical island arc rocks (Fig. 10a) (Sun and McDonough, 1989) rather than MORB or OIB. Chondrite-normalized rare earth element REE patterns for the dikes (Fig. 10b) show strong enrichment of light REE (La-Eu) compared to heavy REE (Gd-Lu, that is also characteristic of an arc setting. This suggests that the dikes and the likely cogenetic gabbro formed in an arc (supra-subduction zone) setting.6.2 Upper Crustal Rocks of the FRB and Implications for Ophiolite Development and Orogenesis
The geochemical affinity of the rare upper crustal (gabbro and dikes), upper plate rocks suggest that oceanic crust development took place in a supra subduction zone environment. High-grade, primarily metamafic metamorphic rocks crop out along much of the length of the FRB and these are characterized by high-temperature, high-pressure metamorphism (Eck, 2013; Masutsubo and Wakabayashi, 2013; Jackson et al., 2011; Smart and Wakabayashi, 2009). The HT-HP metamorphism and metabasite-dominated ocean plate stratigraphy of these rocks, as well as their structural position beneath the ultramafic rocks of the FRB is typical of a metamorphic sole, although the Devil's Gate amphibolite is somewhat thicker (over 1 km thickness). We propose that the amphibolite faces rocks formed in a subdution initiation with a young, hot oceanic crust and then exhumed into upper plate ultramafic belt (Fig. 11a).
The low P burial metamorphism of the upper plate gabbro and dikes along the La Porte Road require significant exhumation of the amphibolite to them (Fig. 11b), typical of relationships of metamorphic soles to upper plate ophiolites (Wakabayashi and Dilek, 2003). However, some of the temporal-metamorphic relationships along the FRB are not typical of upper plate ophiolite-metamorphic sole pairs. The > 100 Ma difference between oldest and youngest high-temperature metamorphic ages are certainly different than typical metamorphic soles that have restricted age ranges (Wakabayashi and Dilek, 2003). In addition, the high-grade metamorphic rocks of the FRB from the Yuba River area (directly south of the La Porte Road area and DGA) display evidence of at least one and possibly two episodes of HT-LP metamorphism in addition to HT-HP metamorphism (Masutsubo and Wakabayashi, 2013; Jackson et al., 2011). The HT-LP metamorphism may be related to ridge subduction events that took place soon after subduction initiation events (Fig. 11b) (Masutsubo and Wakabayashi, 2013). The Calaveras complex formed structually beneath the ultramafic belt and younging toward to the west and Shoo Fly complex formed to the east (Fig. 11c).
Geochronologic relationships associated with other ophiolitic belts in the Cordillera of California show an extended history similar to that of the FRB but with an extended, multi-stage history of formation and emplacement of upper crustal rocks (Saleeby, 1992, 1982). The rock association recorded in the FRB and its long duration of development is similar to the Zhaheba-Aermantai ophiolitic belt in eastern Junggar of the Central Asian orogenic belt (Luo et al., 2017). In contrast to the FRB, the Zhaheba-Aermantai ophiolitic belt includes development of mature arc edifices as well as the preservation of larger amounts of upper crustal oceanic rocks. These complexities illustrate a much greater variety of SSZ ophiolite development and involvement in orogenesis than generally appreciated.ACKNOWLEDGMENTS
We appreciate Brian Windley who has taught us to analyze ophiolitic complexes in the field. We are grateful to two anonymous reviewers for their constructive comments and suggestions. This study was ﬁnancially supported by the National Natural Science Foundation of China (Nos. 40739904, 41072093, and 41302117) and SINOPEC. The final publication is available at Springer via https://doi.org/10.1007/s12583-017-0964-9.
Electronic Supplementary Material: A supplementary material (Table S1) is available in the online version of this article at https://doi.org/10.1007/s12583-017-0964-9.
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