tinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and. (2) biotite-bearing, hornblende-absent leu- cotonalite. Higher total REE and ...
Magmatism and deformation, southern Revillagigedo Island, southeastern Alaska R D COOK • J t' .
| f Department of Geology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010
M. L. C R A W F O R D »
G. I. OMAR Department of Geology, University of Pennsylvania, Philadelphia, Pennsylvania 19104 W. A. CRAWFORD Department of Geology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010
ABSTRACT The mid-Cretaceous, calc-alkaline, LREEenriched plutons on southern Revillagigedo Island, southeastern Alaska, form two distinct groups: (1) biotite-epidote-hornblende tonalite of the Moth Bay pluton and (2) biotite-bearing, hornblende-absent leucotonalite. Higher total REE and lower Na z O, A1 2 0 3 , and Sr distinguish the Moth Bay tonalite from the leucotonalite. The plutons are surrounded by narrow contact metamorphic aureoles containing kyanite and staurolite. Pressure estimates for emplacement based on metamorphic conditions of contact rocks and from igneous amphibole compositions are - 8 - 9 kbar. Detailed analyses of pluton/host rock relationships indicate that pluton emplacement was synchronous with orogen deformation. Preemplacement thrusting placed the metavolcanic and metasedimentary rocks of the Taku terrane structurally above the younger metasedimentary rocks of the Gravina belt. The contact between these units was subsequently cut by the Moth Bay pluton. The southern margin of the Moth Bay pluton has been transformed to blastomylonite. Contact metamorphic textures and undeformed dikes which cut the blastomylonite indicate that shearing did not outlast crystallization of the pluton. A still later phase of deformation transposed the pluton-related structures and generated a northeast-dipping, ductile thrust fault (Southern Revillagigedo shear zone) along the southern shore of Revillagigedo Island. The shear zone and earlier structures all trend approximately northwest. Fission-track age determinations and track-length measurements in apatite were used to determine the cooling histories of the plutonic units. Fission track ages from the western part of the study area are 51.3 ± 4.0 Ma (apatite), 57.8 ± 4.4 Ma (zircon), and 69.8 ± 4.2 Ma (sphene); eastward, toward the Coast batholith, fission track ages are ~10
INTRODUCTION
m.y. younger. These younger ages may result from a tectonic/thermal event associated with batholith emplacement. This same event may have generated north-trending structures that overprint older northwest-trending structures on the east side of the study area.
A geologically complex zone in southeastern Alaska marks the suture between the Alexander terrane and Jurassic North America (Monger and others, 1982; Crawford and others, 1987).
5 5° 3 0 '
KETCHIKAN
AREA S H O W N IN F I G U R E Figure 1. Major geologic and tectonic features of southern southeastern Alaska and British Columbia (modified from Berg and others, 1988). Thin dashed line = terrane boundary, thick dashed line = Southern Revillagigedo shear zone, CRM = Coast Range megalineament, A = Alexander terrane, T = Taku terrane, G = Gravina belt overlap assemblage.
Geological Society of America Bulletin, v. 103, p. 829-841, 12 figs., 4 tables, June 1991. 829
2
N
CANADA
P
830
Formation of this crustal suture involved the collapse of the Jurassic to mid-Cretaceous Gravina basin and associated volcanic arc (Berg and others, 1972) and tectonic juxtaposition with the older volcanic-arc-like units of the Taku terrane (Crawford and others, 1987; Rubin and Saleeby, 1987a, 1987b). Within the deforming crustal rocks, a mid-Cretaceous (100-90 Ma) magmatic arc was established. The plutons representing the deeply eroded remnants of this arc are presently exposed along the length of southeastern Alaska (Brew and Morell, 1980; Cook and others, 1987; Arth and others, 1988; Cook and Crawford, 1990). Although these plutons offer the possibility of constraining the timing of orogeny and the nature of crustal evolution in southeastern Alaska, little has been known about the geochemistry of the plutons and the
COOK A N D OTHERS
relationship between pluton emplacement and deformation of the hosting crustal rocks. In this paper, we discuss the petrology and setting of one syntectonic mid-Cretaceous plutonic suite emplaced into units of the Taku terrane and Gravina belt on southernmost Revillagigedo Island, southeastern Alaska, between George Inlet and Alava Bay (Figs. 1 and 2). GENERAL GEOLOGY The southeastern part of Revillagigedo Island, which is the focus of this study, is composed of imbricated slices of Alexander and Taku terrane units and Gravina belt rocks (Berg and others, 1978, 1988; Rubin and Saleeby, 1987a; Saleeby, 1987). Our mapping suggests a twofold subdivision of the units intruded by the mid-
Cretaceous plutons included in this study. These units are separated by an inferred thrust fault that crosses the area from northwest to southeast (Fig. 2). The presence of the thrust fault is based on age relations discussed below, on contrasts in structural style on each side of the proposed thrust, and on locally developed intense deformation along the mapped thrust trace. In the northern part of the study area, a distinct group of volcanic, calcareous, and siliciclastic rocks can be traced from East Behm Canal across northern Thorne Arm, and Carroll Inlet to George Inlet. Along the northern and eastern shores of Thorne Arm, calcareous siliceous schists are locally interlayered with hornblenderich units. We tentatively interpret these schists as metamorphosed intermediate to felsic tuffs on the basis of (1) their mineral composition
Figure 2. Generalized geologic map of southern Revillagigedo Island (modified from Berg and others, 1988). Triangles mark sample localities discussed in the text. Circled K = kyanite occurrence.
MAGMATISM A N D DEFORMATION, SOUTHEASTERN ALASKA
(quartz + white mica + epidote/clinozoizite + plagioclase + biotite + carbonate ± garnet); (2) possible relic fragmental volcanic textures; and (3) their association with metamorphosed pillow basalts, volcanic breccia, and massive basaltic units along East Behm Canal and Carroll Inlet just north of the study area. These metavolcanic rocks are structurally underlain by a metasedimentary sequence composed of black calcareous schist interlayered with, and underlain by, a distinctive metaconglomerate and quartz-mica schist. The black schist, commonly calcite-rich, contains quartz, muscovite, biotite, minor garnet, and abundant fine graphite. The conglomerate contains siliceous, calcareous, and felsic igneous clasts a few centimeters to 50 cm in diameter in a siliceous and locally calcareous matrix. The rounded to slightly flattened/elongated felsic igneous clasts are of two types: (1) abundant fine-grained felsic clasts, usually with plagioclase phenocrysts; and (2) less common coarse-grained granitic clasts. The contacts
between the black schist and the conglomerate appear to be gradational, although the intense deformation has transposed and largely obliterated primary depositional features. Buff to white dolomite and calcite marble (Fig. 2) are at the structurally lowest level. The coarsely crystalline marble unit varies in thickness along strike and is characterized by numerous rootless folds that indicate the marble behaved in a structurally incompetent fashion. A distinctive siliceous pebble conglomerate lies at the base of the marble along the eastern shore of Thorne Arm and in Alava Bay to the east. In Alava Bay, this conglomerate is underlain in turn by a second volcaniclastic unit. None of the marble outcrops in the study area has yielded fossils. Silberling and others (1981), however, described latest Middle Triassic and late Early Permian fossils from this marble and interbedded phyllite at a locality on the western side of George Inlet just west of the study area. Because of these fossils, Berg and others (1988)
FIGURE 2 EXPLANATION Quaternary and Tertiary
Contacts Inferred
Volcanic rocks
Oligocene
Shear zone
Ketchikan Lakes Pluton
Inferred thrust Cretaceous
hidden
831
assigned the marble and all overlying units to the Taku terrane. Rubin and Saleeby (1987b) reported U / P b ages of 158-154 Ma from coarsegrained igneous cobbles in the conglomerate that structurally overlies the marble. Based on this age, they assigned the conglomerate to the Gravina Belt. We have no evidence to support the complex stratigraphic and structural relations required for this latter interpretation, and therefore we retain the interpretation of Berg and others (1988). In the southern part of the study area, a sequence of gray to black siliceous mica and garnet-bearing schists form the primary host rock for the intrusive rocks that are the focus of this paper. We assign this group of schists to the Gravina Belt. They contrast with the black schists structurally above the Permian marble in a general lack of carbonate minerals. Scattered pods and thin lenses of dark, fine-grained marble are locally present. The contact between the Permian marble and the structurally underlying schist has been mapped as a thrust fault to the west (Berg and others, 1988). We extend this inferred thrust eastward across the study area (Fig. 2) to explain the east-to-west disappearance of units that underlie the Permian marble, and we infer the juxtaposition of Permian rocks over the above units to be no older than Upper Jurassic. The trace of the inferred fault is poorly exposed, and where it is exposed, evidence for a fault is equivocal. The west-northwest-eastsoutheast- striking pluton outcrop belt truncates the northwest-striking thrust fault on the eastern shore of Thorne Arm where the plutons intruded the southern margin of the Permian marble unit.
Moth Bay Pluton í VI
Leucotonalite
Mesozoic Mafic/ultramafic
Complex
Gravina Belt Graphitic schist and minor metavolcanic rocks
Taku Terrane Volcanic rocks WiM
Calcareous and siliceous metasediments, minor metavolcanic rocks Metaconglomerate
STRUCTURE
Marble
The record of deformation preserved in the metamorphic rocks of the study area and in the mid-Cretaceous plutons documents pluton emplacement during shortening and tectonic thickening of the region. The metamorphic rocks
• • ° n*
i' i'
Along the southern shore of Revillagigedo Island (between Lucky Cove and Point Alava, Fig. 2) marble and siliceous muscovite-bearing schist are interlayered with hornblende-bearing calcareous schist (garbenschiefer). These rocks are separated from the structurally overlying Gravina Belt schists by a 30°-50° northeastdipping, ductile thrust fault (Crawford and Crawford, 1986) herein named the "Southern Revillagigedo shear zone." The rocks in the lower plate of the thrust are tentatively assigned to the Alexander terrane, because they can be traced to the east and southeast along the eastern (mainland) shore of Revillagigedo Channel (Fig. 1), where they lie adjacent to Alexander terrane mid-Paleozoic rocks.
Alexander Terrane (?) Marble and siliceous schist
COOK A N D OTHERS
832
surrounding the plutons are distinctly foliated. This foliation is defined by alignment of mica flakes and amphibole needles, suggesting that the fabric was produced during regional metamorphism. North of the belt of plutons, and especially within the calcareous schists and siliceous metavolcanic Taku units that structurally overlie the Permian marble unit, all early features, including bedding and scarce early isoclinal folds, are strongly transposed parallel to the foliation. In the conglomerates, siliceous and calcareous fragments are both flattened and stretched, with their long axes parallel to the regional mineral and crenulation lineations (Fig. 3 A). South of the inferred thrust fault, the earlier metamorphic foliation is folded into tight-toopen folds of as much as 5-m amplitude that plunge gently to the east (Figs. 3B, 3C). This folding produced a weakly to moderately developed axial planar fracture cleavage, the result of alignment of the limbs of millimeter-sized crenulations. Locally, white mica and biotite lie parallel to the axial planes of these upright folds. The schists were deformed and metamorphosed prior to emplacement of the plutons. Foliated xenoliths of country rock are enclosed in the southern margin of the Moth Bay pluton. Adjacent to the igneous bodies, the schists attain a massive to flinty appearance in response to contact metamorphism. Abundant garnet, some staurolite, and at three localities kyanite (K in Fig. 2) grew randomly over an earlier foliation. At several localities, aplite dikes, apparently associated with the neighboring plutons, intruded parallel to the axial surfaces of the upright folds of the foliation. Where the foliation in the schists remains unfolded, the aplitic dikes cut that foliation at a small angle. We infer from the relationship between the dikes and the foliation that the folding accompanied pluton emplacement.
Figure 3. Structural domain map and equal-area projections of structural elements in the study area. Lettered areas on the map show the locations where the structural data shown on the projections were collected. In all projections except D, the poles to foliations, layers, and fold axial planes coincide and have been contoured together by the Schmidt technique using the computer program SPLOT (Darton Software). In each figure, contours are 0,3%, 6%, and >9% points per 1% area. X = crenulation and mineral lineations, + = fold axes.
Across much of the Moth Bay pluton (Fig. 2), a variably oriented, moderate to strong foliation is defined by aligned biotite, hornblende, epidote, plagioclase, and discoidal hornblende-rich enclaves. Well-preserved igneous textures and lack of deformation of individual mineral grains
TABLE 1. REPRESENTATIVE MODAL ANALYSES
Mafic Moth Bay pluton
Sample no.
PI Qtz Hbl Bt Ep Kfs Grt Opaq Spn Ap Aln Zrn Chi Ms
Felsic Moth Bay pluton
Leucotonalite
MB1
MB2
MB7
MB8
MB4*
MB9
LT1
LT2
LT3
32 11 30 3 15
39 12 19 13 16
38 13 26 8 13
57 20 1 13 8
64 16 4 7 7 2
46 24 5 10 8 8
64 16
46 26
51 18
-
-
-
2 9
9 11
8 10
-
+
+
-
-
-
+
+
+
+
+ +
+
•
-
+
-
-
+
+
•
*
+
+
+
+
+
+ +
+ +
+ +
+ +
+ +
+ +
+
+
+
+
+
+
+
+
2 6
-
+
-
+
+
+
+
2
1
+
+
+ +
+
-
9
8
•
3
Note: PI = plagioclase, Qtz - quartz, Hbl = hornblende, Bt = biotite, Ep = epidote, Kfs - potassium feldspar, Grt = garnet, Opaq - opaque mineral, Spn = sphene, Ap - apatite, Aln = allanite, Zrn = zircon, CM = chlorite, Ms = muscovite. Chlorite and muscovite are secondary minerals. Epidote is mostly secondary in LT1,2,3. + = trace amount, - = not observed, • • not determined. Modes are based on 1,200 counts per sample. •From Arth and others (1988) sample AK1.
suggest that the foliation is a result of flow during magma emplacement. At the southern margin of the pluton, the trends of foliations and lineations within the pluton (Fig. 3D) parallel the fabric in adjacent country rocks (Fig. 3C). The degree of solid-state deformation of plutonic rocks increases as the margin is approached. At the northern end of Cone Island (Fig. 2), igneous textures are dominant, but quartz and plagioclase are slightly recrystallized at grain boundaries. The pluton margin at the southern end of Cone Island is a several-meterwide blastomylonite zone. The structures in this zone reflect local deformation accompanied by, or followed by, pluton emplacement. Undeformed dikes that originate in the pluton, however, cut the deformed pluton margin and adjacent schist. These dikes, and randomly oriented kyanite in the contact metamorphic aureole, demonstrate that pluton crystallization and thermal effects locally outlasted deformation. Taken together, the structural relations between the Moth Bay pluton and the enclosing country rocks suggest that deformation was synchronous with pluton emplacement. The Southern Revillagigedo shear zone lies south of the plutons and separates black schist
MAGMATISM AND DEFORMATION, SOUTHEASTERN ALASKA
Figure 3. (Continued). A. Data from metasedimentary rocks northwest Thorne Arm and adjacent parts of Carroll Inlet. + = fold axes and long axes of pebbles. 79 points. B. Data from schists along the east shore of Carroll Inlet south of the area represented by A and north of the belt of intrusive igneous rocks. The girdle pattern reflects folding of the foliations and layers contoured in A. 79 points. C. Data from schists south of the Moth Bay pluton both west and east of Thorne Arm. 31 points contoured. D. Foliations (triangles) and mineral lineations (X) in Moth Bay pluton at Cone Island. Structural trends are identical to those in the adjacent schists (C). E. Structures at Lucky Cove at the northern margin of the Southern Revillagigedo shear zone. The contoured foliations, layers, small shear zones, and the fold axes show the reorientation of structural elements from the east-west trend adjacent to the plutons (C) to the northwest-southeast trend of the shear zone (F). The box and great circle are the average trend of the fold axes and the plane perpendicular to that trend. 23 points. F. Southern Revillagigedo shear zone structures from Lucky Cove, eastward to Point Alava. 28 points. G. Structural data from northeast and eastern Thorne Arm and north of Alava Bay. The pattern of the contoured layers and foliations reflects folding of the planar elements along northwestern Thorne Arm (A) about a gently north- or south-plunging fold axis (+). 52 points.
assigned to the Gravina belt from structurally underlying marble and calcareous siliceous schist. Numerous plagioclase porphyritic felsic sills lie within the shear zone. Most of the sills and segments of pluton margins in the shear zone are transformed into a medium- to finegrained blastomylonite. Sills a few meters thick are stretched and boudinaged parallel to the strike of the foliation of the enclosing schists. Deformed sills are well foliated, with coarse muscovite and biotite flakes and quartz lenses that symmetrically wrap around subrectangularto-ovoid plagioclase grains. A few less-deformed sills retain their original igneous textures, suggesting either that they intruded slightly later than the main shearing episode or that deformation was partitioned into the adjacent more easily deformed micaceous schists. Fabric elements that serve as kinematic indicators in igneous rocks in the shear zone, such as broken and
833
N
rotated plagioclase crystals and asymmetric recrystallized tails on plagioclase porphyroclasts, suggest a north-over-south sense of shear. In the schists deformed by the Southern Revillagigedo shear zone, well-developed shear bands, small folds, and asymmetric pressure shadows around garnet indicate the same sense of shear as recorded by the deformed igneous rocks. The Southern Revillagigedo shear zone cuts upright folds and other structures formed during pluton emplacement. This is especially obvious
in the vicinity of Lucky Cove where fine siliceous black schist is thrust over highly deformed siliceous marble. At the top of the shear zone, layering in the metasedimentary rocks and the older foliation are folded and cut by small shears a few meters long. Both the folds and small shears trend northwest, (Fig. 3E) at a distinct angle to the east-west-trending folds in the overlying siliceous black schist (Fig. 3C). The slight discordance between the shear zone (Figs. 3E, 3F) and earlier structures north of the belt of
834
COOK A N D OTHERS TABLE 2. MINERAL RIM COMPOSITIONS FOR GEOTHERMOBAROMETRY
Plagioclase Sample
Si02 Ti02 AI 2 O 3 FeO* MnO MgO CaO Na z O
37.75 21.58 30.21 1.76 2.99 6.10
37.20 0.02 21.08 37.20 2.35 2.85 5.49
K,0
100.39
12.98
37.58 1.58 19.53 16.31 0.07 11.85
8.36 94.57
0.37 8.46 95.76
37.87 1.41 18.50 15.45
99.04
58.00
60.47
46.88
26.65 0.44
25.12
35.06 1.47
7.76 6.90
6.53 7.87 0.04 100.04
0.84
99.75
1.36 8.56 94.55
Note-, samples 1 and 2 are from the vicinity of Carroll Point (Fig. 2). •Total iron as FeO.
Figure 4. Modal composition of igneous rocks based on plutonic rock classification of Streckeisen (1973). Circles = Moth Bay pluton; squares = leucotonalite.
plutons (Fig. 3A) suggests that the regional stress-field orientation did not change markedly between the development of that earlier foliation and associated thrust fault and the subsequent deformation that created the Southern Revillagigedo shear zone. Inasmuch as the Southern Revillagigedo shear zone cuts the plutons and associated structures, much of the deformation must have postdated pluton emplacement. Additional evidence for the timing of shearing derives from the metamorphic mineral fabric. At some localities within the shear zone, especially south of Carroll Point, garnet has grown over the blastomylonitic fabric in sills deformed within the shear zone, showing that deformation occurred while the rocks were at least at upper-greenschist facies conditions, possibly during the waning stages of the contact metamorphism associated with pluton emplacement. Continued deformation at lower temperatures is suggested by other rocks in which the products of retrograde alteration, chlorite, muscovite, and epidote are sheared. In the northeast part of the study area, the structures change markedly in style and orientation. More or less upright, north-trending, tight folds predominate and transpose and thus postdate the above structures (Fig. 3G). PETROLOGY OF MID-CRETACEOUS PLUTONIC ROCKS
Fe3 5 . 0 ; T i < 0 . 5 0 ; 0.75 0 . 7 0-
pargasitic hornblende
The mid-Cretaceous igneous rocks are separated into two petrographically distinct units: (1) equigranular hornblende and magmaticepidote-bearing tonalites, the largest of which is the 101 Ma (Rubin and Saleeby, 1987a) Moth Bay pluton; (2) undated, biotite-bearing, hornblende-absent leucotonalites. Both rock types occur as sills and discordant tabular plutons. No mutual contacts between these igneous bodies are exposed, and the relative timing of their emplacement is not known. Recognition of systematic petrologic and chemical variation in individual bodies is hindered by limited exposure, which is mostly confined to shoreline outcrops.
pargasite O MB7 O MB2 0
ferroan pargasitic hornblende
ferroan-pargasite
O
o> 5
•
O •
MB10
The Moth Bay pluton consists primarily of biotite-epidote-hornblende tonalite (classifica-
o
A
D 0" 0
