n Alas - Alaska Division of Geological & Geophysical Surveys - State ...
Share Embed Flag
Download ePaper

n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

n Alas - Alaska Division of Geological & Geophysical Surveys - State ... n Alas - Alaska Division of Geological & Geophysical Surveys - State ...

dggs.alaska.gov
from dggs.alaska.gov More from this publisher
10.04.2013 Views

0 @ KILOMETERS EXPLANATION - UIRd \sfand and Undsnburp MnsJa plmns -- - Figmz 80.-Pete :rsburg quadrangle, showing approximate lcrct htions of Cretaceous and Cretaceous(?) graw itic -. >. plutc >ns. Numbers refer to A granitic Wies descrl~ea In text and figure 81. anlledral and crystallized interstitially. Sphene and apatite are common accessory minerals. The color index for these rocks ranges from 17 to 27. The margins of stocks cornping body 2 are ~haracterized by fine-grained matic minerals and (garnett uscovite-epidote-biotite eplite dikes of granltjc 3rnposition. Body 2, on northern Mitkof Island and the east st of the Lindenberg Peninsula, is mIneralogically milar to body 1, although it is generally more afic. The color index ranges from 32 to 52. Mafic inerals generally are more clustered together, and the plagioclase phenacrysts are smaller than those in body I. The rocks are crowded glagioclass-porphycitic quartz monediorite and quartz diorite. Body 3, on southern Mitkof Islrtnd, also has the crowdedglagioclase texture, although it B not everywhere porphyritic. Parts of this body ere inequigranular and contain serlate plagioclase. The mineralogy differs in that biotite and quartz are more abundant than in body 1. Hornblende is the dominant mafic mineral, although biotite is more prevalent toward the center of several stocks. Compositions of these rocks range from quartz monzodiosite to tonallte, and their color indices from 30 to 49. Aplite dikes, similar to those of body 1, are present in the contact zones of the southern part of this body. EWy 4, on Mitkof Island, differs texturally from the crowded-plagioclese unit. Most of these rocks are 4 Bady 1 0 Body 2 + Body 3 a Body 4 Pigure 81.-Madal compositions of granitic rocks of Cretaceous and cretaceous(?) age from the Lindenberg Peninsula and MItkof Island (see fig. 80 for locations). DI, diorlte; GD, granodiorite; MD, monWiorite; QD, quartz dlorite; Q0, quartz monzodiorite; Td, tonalite.

foliated and inequigranular end contain seriate plagioclase, although locaI variation to porphyritic as well as equigranular texture occun. The mineralogy of this body is the same as that of the crowdedplaglbcIase bodies. Biotite and hornblende are the dominant maf ic minerals, although their proportions vary. Primary epidote, sphene, and apatite are also present. The composition of body 4 range from granodjorite to tonalite, and the color index from 24 to 35. Rocks from the east margin contain anhedral redorange garnets associated wlth the mafic minerals. Body 5, on central Mitkof Island, is anomalous in comparison with the surrounding stocks. Hornblende Is generally the dominant mafic mineral, although biotite is locally present in equal amounts. Hornblende occurs as anhedral crystals, commonly clustered togeLher. In about half the rocks studied, the hornblende has relict pyroxene cores, and much of it is poikilitic, with enclosed quartz, feldspar, and opaque minerals. Primary epidote is a minor phase in some rocks. Phgioclase, as small twinned and zoned laths grown together, forms the bulk of the gmundrnm. These rocks are inequigranuhr foliated rnonzodiorfte to quartz diorlte; the color index rang- from 34 to 55. An arc of inequigranular crowded-plagloclase quartz rnoneodiorite and quartz diorite crops out along the north edge of this body. Body 6; on the northern Lindenberg Peninsula, is much like body 5. The rocks ere mostly foriated inequigranular wtz monaodiorite, and the color index ranges from 52 to 51. Hornblende, the dominant maflc mineral, ranges from small &mete rmbhedral crystals to larger anhedral crystals, wlth inclusions of quartz and feldspar and commonly relict pyroxene cores. Primary epidote L present in very smd amounts. Plagioclase is twinned and Boned, and commonly exhbits a seriste texture. Sphene and apatite are common accessory minerals. Mapping is incomplete southeast of Mitkof Island, and the plutons there have been compared with the Mitkof Island and Lindenbem Peninsula ~lutons by reconnaissance mappis only. %cks observed in the southeastern Dart of the area. include epidote(sarnet)hornblende-bibtite graflodiorite, ton&e, and-qwtz diorite with ineguigranular to porphyritic textures. A recent potassium-argon age determhation on hornblende from body 4 yielded an age of 80.1 m.y. (Marvin hnphere, written commun., 1982). Bodies 1 through 4 we correlated with similar porphyritic and inequigranular rocks in the Bradfield Canal 1:250,000scale quadrangle, which were also dated at approximately 90 m.y. by potassium-argon methods (R. D. Koch, oral commun., 1982). The Mitkof Island and Lindenberg Peninsula plutons may represent a complex intrusive sequence. Bodies 1 through 4, though exhibiting Iimited textural differences that range from crowded porphyry to inequigranular, have similar mineralogles and ate likely to be coeval; they are herein considered to constitute a distinct group. Bodies 5 and 6 compose a second distinct group; pbkilitic hornblende with pyroxene cores, an ineguigranular texture, and a plagkalase-dominant ground mass distinguish them from bodies 1 through 4. Despite these observed differences, however, the presence of primary epidote and the generally similar compositions and structural settings indicate that these two groups of plutonlc bodies may be genetlcaUy related. BBFERENCE CITED Brew, D. A., and Morrell, R. P., 1980, Intrusive rooks and plutonic belts of Wutheastern Alaska, U.S.A .: U.S. Geological Survey Open-Pile Report 80-18, 34 p. Late Oligocene gabbta near Ketehilrsn, m u ~ e m Alaska Bp Richard D. Koch aod Bsymond L. Klliott A large elongate gabbro complex, here referred to informdy as the gabbro of Ketchikan takes, is exposed over an irregular area, 7 by 16 km, in the vicinity of Ketchikan Lakes just north and east of the town of Ketchikan (area 8, fig. 72; fig. 82). Originally believed to be mainly granodiorite and quartz dlorite with a small area of leucogabbro (Berg and others, 1978), this heterogeneous pluton consists enthely of various gabbrofc rocks. Composition varies considerably across this complex, with a crude eonation, with a small slightly offcenter core area oi olivine-bearing twbpycoxene gabbro, wholly enclused by an area of biotite- hornblende-twepyroxene gabbro. A third, discontinuous zone of quartz-bearing gabbro underlies two large areas at the northwest and southeast ends of the complex. A few samples from this outermost zone also contain minor amounts of potassium feldspw. Compositions vary within, as well as among, these zones. Contacts between the zones were not observed in the fleld and are presumed to be gradational. Quartz-free gabbmlc rocks from the ollvhe- bearing core zone and from much of the surrounding mea contain biotite, hy persthene, hornblende, and auglte. The color index for these rocks ranges from about 15 to 50 and is generally 30 or less; thus, the rocks are lagely leucogabbro. Relative abundances of the rnallc miner- vary considerably from place to place; clho yroxene exceeds orthopymxene tn all but one sample P norite). The color index for the quartz-bearing gabbro ranges from about 12+ to 55 and is mostly less than 30 Oeucogabbro). Many of these quartz-bearing rocks contain clinopyroxene, hornblende, biotite, and little or no orthopgroxene; a few contain only hornblende and bIotIte as rnafic minerals. Most of the gabbro is massive, tine to medium gained, serlate to relatively equigrandar, with a hypidlomorphic-grmular texture. Foliation, formed by alinement of plagimlase and mafic minerals, is poorly developed in a few places, and diabasic and rarer aUotriomorphl~anular textures also occur. Quartz, where it occurs, forms small interstitial grains hat constitute less than 5 percent of the rock. Potassium feldspar ~eurs in only a few places, 8s smell untwlnned anhedral interstitial grains. F'lagioolase forms anhedral to euhedral crystah, but the texture of almost all the gabbro 18 dominated by subhedral phgioclase as equant to, more commonly, elongate laths with a length-to-width ratio of 411 to

0 @ KILOMETERS<br />

EXPLANATION<br />

- UIRd \sfand and Undsnburp MnsJa plmns<br />

-- -<br />

Figmz 80.-Pete :rsburg quadrangle, showing<br />

approximate lcrct htions <strong>of</strong> Cretaceous and<br />

Cretaceous(?) graw itic -. >. plutc >ns. Numbers refer to<br />

A<br />

granitic Wies descrl~ea In text and figure 81.<br />

anlledral and crystallized interstitially. Sphene and<br />

apatite are common accessory minerals. The color<br />

index for these rocks ranges from 17 to 27. The<br />

margins <strong>of</strong> stocks cornping body 2 are ~haracterized<br />

by fine-grained matic minerals and (garnett<br />

uscovite-epidote-biotite eplite dikes <strong>of</strong> granltjc<br />

3rnposition.<br />

Body 2, on northern Mitk<strong>of</strong> Island and the east<br />

st <strong>of</strong> the Lindenberg Peninsula, is mIneralogically<br />

milar to body 1, although it is generally more<br />

afic. The color index ranges from 32 to 52. Mafic<br />

inerals generally are more clustered together, and<br />

the plagioclase phenacrysts are smaller than those in<br />

body I. The rocks are crowded glagioclass-porphycitic<br />

quartz monediorite and quartz diorite.<br />

Body 3, on southern Mitk<strong>of</strong> Islrtnd, also has the<br />

crowdedglagioclase texture, although it B not<br />

everywhere porphyritic. Parts <strong>of</strong> this body ere<br />

inequigranular and contain serlate plagioclase. The<br />

mineralogy differs in that biotite and quartz are more<br />

abundant than in body 1. Hornblende is the dominant<br />

mafic mineral, although biotite is more prevalent<br />

toward the center <strong>of</strong> several stocks. Compositions <strong>of</strong><br />

these rocks range from quartz monzodiosite to<br />

tonallte, and their color indices from 30 to 49. Aplite<br />

dikes, similar to those <strong>of</strong> body 1, are present in the<br />

contact zones <strong>of</strong> the southern part <strong>of</strong> this body.<br />

EWy 4, on Mitk<strong>of</strong> Island, differs texturally from<br />

the crowded-plagioclese unit. Most <strong>of</strong> these rocks are<br />

4 Bady 1<br />

0 Body 2<br />

+ Body 3<br />

a Body 4<br />

Pigure 81.-Madal compositions <strong>of</strong> granitic rocks <strong>of</strong><br />

Cretaceous and cretaceous(?) age from the Lindenberg<br />

Peninsula and MItk<strong>of</strong> Island (see fig. 80 for locations).<br />

DI, diorlte; GD, granodiorite; MD, monWiorite; QD,<br />

quartz dlorite; Q0, quartz monzodiorite; Td, tonalite.

logo

products

Resources

Company

Terms of service
Privacy policy
Cookie policy
Cookie settings
Imprint
Change language
Made with love in Switzerland
© 2024 Yumpu.com all rights reserved

Hooray! Your file is uploaded and ready to be published.

Saved successfully!

Ooh no, something went wrong!