Description of Rock Units, Type Area for Carpenter Creek Metamorphic Complex (See Map)

Description of Rock Units, Type area for Carpenter Creek Metamorphic Complex (see map)

[Text from Pavlis, 1986b with minor modifications and update notes]

Jum: Mafic-Ultramafic bodies: Tectonite ultramafic rocks, and cumulate ultamafic and mafic rocks are exposed in two large lensoidal masses near the topographic divide between Wolverine and Carpenter Creek. Together these fault bounded bodies constitute the Wolverine ultramafic complex of Clark (1972). [revised terminology here: Wolverine mafic-ultramafic suite] Similar rocks also occur in lower Jim Creek but in that locality ultramafic rocks occur only along a narrow band at the southern edge of a body composed predominantly of layered gabbro. Ultramafic rocks (talc-calcite schists) also occur as lenses within the “red mélange” unit of Pavlis (1983).

In all three mafic-ultramafic bodies a conspicuous layering dips to the south at shallow to steep angles. The layering varies from macroscopic interlayering of gabbro and peridotite to mesoscopic interlayering of light and dark bands, pyroxene rich bands or thin chromite layers. A small area on the east-southeast margin of the largest ultramafic body is underlain by tectonized ultramafic rocks showing isoclinal folded chromite bands and preferred orientation of minerals. [Note: these rocks were later studied by Mehl et al. (2003) who confirmed this observation through petrofabric studies] Most of the rocks, however, do not show evidence of ductile deformation and many show cumulate textures. Thus, most of the rocks probably represent parts of a cumulate igneous complex. Dunite, generally with thin (1mm to about 5cm thick) bands of chromite, is the most abundant lithology in both ultramafic bodies of the Wolverine complex. Layered gabbro and clinopyroxene rich peridotites (originally wehrlites and[or] lherzolites) appear to be in about equal proportions in the largest ultramafic body but gabbros are absent in the smaller ultramafic body in upper Carpenter Creek (type area shown here).

All three ultramafic-mafic bodies have been metamorphosed at greenschist to amphibolite facies. Mafic rocks are generally completely recrystallized to amphibole + zoisite +/- chlorite but a few rocks retain corroded clinopyroxene and plagioclase. Ultramafic rocks contain abundant remnant phases but show replacement patches of chlorite + tremolite, talc, tremolite alone, or talc + chlorite +/- calcite. Serpentinization is ubiquitous at the margins of the ultramafic bodies but aside from shear zones, serpentinite is generally absent within the ultramafic complex other than surficial weather products.

The serpentine free paragenesis is interpreted as the product of hydrothermal metamorphism that was controlled by fracture permeability and involved marked mass transport in the fluid phase. The serpentine free paragenesis is not associated with penetrative deformation suggesting that either the metamorphic peak was post-kinematic or the ultramafic bodies behaved as rigid pods with deformation limited to internal shear zones (Pavlis, 1983). Serpentinites are of two types: 1) blocky, black or dark green, highly slickensided rock with serpentine growing largely as replacement patches; and 2) green to blue-green serpentinite schist with 1-10cm entrained blocks of less altered ultramafic rock. The former occur along the margins of the bodies and locally show evidence that the serpentization and shearing post-date metamorphic tremolite and (or) chlorite. The latter, however, occur largely in shear zones within the ultramafic masses and their age relationship to serpentine free metamorphic assemblages is unclear. a: mafic schist: The most abundant metamorphic rock type of the Carpenter Creek metamorphic complex is a group of variably foliated mafic schists. The mafic rocks are compositionally indistinguishable in the field but metamorphic grade varies from upper greenschist facies to amphibolite facies [Note: subsequent work by Barnet et al. (1994) confirmed this petrographic observation with quantitation thermobarometry]. In the field the rocks were divided into a greenschist unit (g) and an amphibolite unit (a).

Although they were not divided during mapping, the mafic schists are of two types: a) banded amphibolites and(or) greenschists in which mafic bands are interlayered with 1-10cm thick bands of calc-silicate; and 2) massive, foliated amphibolite and greenschist. In amphibolite facies rocks the calc- silicate bands in the banded amphibolites contain diopside + garnet + plagioclase +/- zoisite +/- calcite. In lower grade rocks, however, much carbonate remains with only zoisite as the secondary mineral.

The mafic schists also contain intercalated bands and (or) lenses of pelitic schist which vary in size from thin 1cm thick bands to layers several meters in thickness, but too small to map at 1:25,000. In higher grade rocks these bands commonly are infolded with amphibolite in isoclinal folds formed during the main ductile deformation. In lower grade rocks, however, similar argillaceous bands appear to be primary or a result of pre-metamorphic structural complications [Note: this 1986 observation is consistent with the interpretation of this paper that these rocks were originally a mélange, prior to metamorphism and ductile deformation]

[Aside for readers of this document: In the type area described here the unit is exclusively the amphibolite unit with banded amphibolites dominant and abundant bands and infolds of schist. Many of the mapping details of this unit were lost in the map compilations of Burns et al. 1991, and Pavlis was not given an opportunity to edit problems in the map of this type, which led to loss of these mapping details in published work. This problem was further complicated when the 1:25,000, more detailed compilation map of the area was lost by the Alaska Division of Geological and Geophysical Surveys. Thus, the only remnants of this more detailed mapping are in field map sheets; available from T. Pavlis] qfs: Quartzofeldspathic schist and q: quartzite: Scattered throughout the Carpenter Creek metamorphic complex, and prominent in this type area, are a series of quartz-rich metasedimentary rocks. The largest single exposure of this unit is along the upper divide between the south and north forks of Wolverine Creek (west of this type area). Included in this assemblage are a variety of metasedimentary rocks but the characteristic lithology is a mesoscopically laminated schist in which 1m to 2cm thick quartzite bands are interleaved with mica schists. In the type area this assemblage is divided into a schist dominant assemblage (unit qfs) and a quartzite unit (q) based on relative abundance of quartzite (in unit q, >50% of the rock in quartzite). The mica schists are mostly garnetiferous biotite schist but many of the rocks also carry muscovite whereas others carry hornblende. The latter are generally quartofeldspathic and are similar to the “brown schist” (JPbs) unit mapped in upper Wolverine Creek by Pavlis (1986b). [Note: W79-28a analyzed in this study is one of these quartzofeldspathic schists and confirms the conclusion these rocks are almost certainly metagreywackes] The quartzites are generally micaceous but most are not flaggy in outcrop. Some quartzites are calcareous and carry unusual calc-silicate assemblages (e.g. diopside, wollastonite, and garnet). The protolith for the quartz rich mica schists is almost certainly a sequence of bedded cherts and argillites (Pavlis, 1983). This conclusion is consistent with observations of lower grade rocks in Jim Creek where similar lithologies are clearly chert-argillite assemblages. m: marble: Although not differentiated on the map, marbles occur at scattered localities throughout the Carpenter Creek metamorphic complex, and are present in the type area, particularly in fault bounded sliver near the western edge of the map. The marbles are restricted to discontinuous thin bands (generally less than 10m thick) or lenses within other units. The marbles are most abundant in the quartz-rich schist but also occur sporatically within mafic units. Most of the marbles are nondescript, gray, laminated rocks with limited evidence of growth of secondary, metamorphic minerals; usually only tremolite as a secondary mineral.