Discovery of a Tertiary granite pluton, i0 Ab98 northern ... · University of British Columbia...
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Discovery of a Tertiary granite pluton, northern Victoria Land EDMUND STUMP, JOHN R. HOLLOWAY, and SCOTT C. BORG Department of Geology Arizona State University Tempe, Arizona 85281 RICHARD LEE ARMSTRONG Department of Geological Sciences University of British Columbia Vancouver, British Columbia V6T 1W5 Canada During the 1980-1981 field season our party undertook sys- tematic collection of early-middle Paleozoic granitoids throughout northern Victoria Land (Stump et al. 1982). In the course of a subsequent strontium isotope study, we discovered that one intrusion is Tertiary. This unique pluton is exposed for about 5 kilometers along the southwestern side of Mariner Glacier, immediately to the east of the confluence of Meander Glacier (figure), where it intrudes Bowers Supergroup at a steep angle, showing very little contact effects. The rock is a medium-grained granite with numerous miarolitic cavities, suggesting a shallow level of emplacement. It is composed of a secondary assemblage of quartz, K-feldspar, albite, chlorite, and accessory amounts of allanite and magne- tite. Microprobe analysis indicates that the K-feldspar is 90 6/El 0 MALTA \ ), PLATEAU C N C1 Tertiary P/u ton -- C 7 Location map of Tertiary-aged granite, Mariner—Meander Glaciers area. percent orthoclase, 10 percent albite (0rq i Ab i0 ), that the albite is Ab98 and that the chlorite is iron-rich. Rims of the magnetite are 95 percent megnetite, 5 percent ulvospinel (MtqUvsp), but their cores are Mt 70 Uvsp 30 , perhaps indicating primary and sec- ondary phases of this mineral. The original assemblage appears to have contained two feld- spars, plus quartz. The chlorite presumably was biotite. The allanite apparently has been undisturbed by secondary effects. The granite has a minimum melt bulk composition with 75 weight percent silica (Si0 2 ) and very low magnesium oxide (MgO) and calcium oxide (CaO). It is slightly peraluminous with 0.9 weight percent normative corundum. The data from the rubidium/strontium (Rb/Sr) analyses are summarized in the table (X = 1.42 x 10 11 for Rb). Although the Analytical data Initial 87 Rb 87 Sr Sample Sra Rb Age' 87S 86 S 86S 865r ETTC 16.2 218 39.1 0.7195 whole rock 9 ± 1 0.7148 ETT 43.4 179 11.93 0.7162 chlorite a In parts per million. In millions of years. "ETT" denotes sample number. date of 9 ± I million years may not represent the primary crystallization of the magma, there is no doubt that this rock is of Tertiary age and not a part of the Cambro-Ordovician Granite Harbor Intrusives or the Devonian Admirality Intrusives. Even if we assume a lowest possible initial ratio (0.704) the age of the whole rock cannot exceed 28 million years. The high initial Sr ratio (0.715) may indicate that the source of the magma was partially melted lower crust, with little or no contamination from mantle basalt. Alkaline basalts and trachytes of the McMurdo Volcanics, locally part of the Hallett Volcanic province, crop out at numer- ous localities from Cape Adare to Mount Melbourne (Hamilton 1972; Nathan and Schulte 1967, 1968; Riddols and Hancox 1968). Locally much of the Malta Plateau, across Mariner Glacier from the Tertiary pluton, is bordered by these volcanics. Dates on McMurdo Volcanics throughout northern Victoria Land range from less than 100,000 years to 8 million years, with one ques- tionable date of 15 million years in the Nathan Hills (Armstrong 1978; Kreuzer et al. 1981). It seems likely that the McMurdo Volcanics provided heat for generation of the granite melt, but the source regions for the mafic and silicic magmas may have been distinct (mantle vs. lower crust). This research was supported by National Science Foundation grant DPP 80-19991. 1983 REVIEW 17
Transcript of Discovery of a Tertiary granite pluton, i0 Ab98 northern ... · University of British Columbia...
Discovery of a Tertiary granite pluton,northern Victoria Land
EDMUND STUMP,JOHN R. HOLLOWAY,
andSCOTT C. BORG
Department of GeologyArizona State University
Tempe, Arizona 85281
RICHARD LEE ARMSTRONG
Department of Geological SciencesUniversity of British Columbia
Vancouver, British Columbia V6T 1W5Canada
During the 1980-1981 field season our party undertook sys-tematic collection of early-middle Paleozoic granitoidsthroughout northern Victoria Land (Stump et al. 1982). In thecourse of a subsequent strontium isotope study, we discoveredthat one intrusion is Tertiary.
This unique pluton is exposed for about 5 kilometers alongthe southwestern side of Mariner Glacier, immediately to theeast of the confluence of Meander Glacier (figure), where itintrudes Bowers Supergroup at a steep angle, showing verylittle contact effects.
The rock is a medium-grained granite with numerousmiarolitic cavities, suggesting a shallow level of emplacement.It is composed of a secondary assemblage of quartz, K-feldspar,albite, chlorite, and accessory amounts of allanite and magne-tite. Microprobe analysis indicates that the K-feldspar is 90
6/El 0
MALTA
\),PLATEAU
C NC1
TertiaryP/u ton
-- C 7
Location map of Tertiary-aged granite, Mariner—Meander Glaciersarea.
percent orthoclase, 10 percent albite (0rq iAb i0), that the albite isAb98 and that the chlorite is iron-rich. Rims of the magnetite are95 percent megnetite, 5 percent ulvospinel (MtqUvsp), buttheir cores are Mt70Uvsp30 , perhaps indicating primary and sec-ondary phases of this mineral.
The original assemblage appears to have contained two feld-spars, plus quartz. The chlorite presumably was biotite. Theallanite apparently has been undisturbed by secondary effects.
The granite has a minimum melt bulk composition with 75weight percent silica (Si0 2) and very low magnesium oxide(MgO) and calcium oxide (CaO). It is slightly peraluminouswith 0.9 weight percent normative corundum.
The data from the rubidium/strontium (Rb/Sr) analyses aresummarized in the table (X = 1.42 x 10 11 for Rb). Although the
Analytical data
Initial87 Rb87 SrSampleSraRb Age'87S 86S 86S 865r
ETTC16.221839.10.7195whole rock
9 ± 10.7148ETT 43.417911.930.7162
chlorite
a In parts per million.In millions of years."ETT" denotes sample number.
date of 9 ± I million years may not represent the primarycrystallization of the magma, there is no doubt that this rock isof Tertiary age and not a part of the Cambro-Ordovician GraniteHarbor Intrusives or the Devonian Admirality Intrusives. Evenif we assume a lowest possible initial ratio (0.704) the age of thewhole rock cannot exceed 28 million years. The high initial Srratio (0.715) may indicate that the source of the magma waspartially melted lower crust, with little or no contaminationfrom mantle basalt.
Alkaline basalts and trachytes of the McMurdo Volcanics,locally part of the Hallett Volcanic province, crop out at numer-ous localities from Cape Adare to Mount Melbourne (Hamilton1972; Nathan and Schulte 1967, 1968; Riddols and Hancox 1968).Locally much of the Malta Plateau, across Mariner Glacier fromthe Tertiary pluton, is bordered by these volcanics. Dates onMcMurdo Volcanics throughout northern Victoria Land rangefrom less than 100,000 years to 8 million years, with one ques-tionable date of 15 million years in the Nathan Hills (Armstrong1978; Kreuzer et al. 1981).
It seems likely that the McMurdo Volcanics provided heat forgeneration of the granite melt, but the source regions for themafic and silicic magmas may have been distinct (mantle vs.lower crust).
This research was supported by National Science Foundationgrant DPP 80-19991.
1983 REVIEW 17
References
Armstrong, R. L. 1978. K-Ar dating: Late Cenozoic McMurdo VolcanicGroup and dry valley glacial history, Victoria Land, Antarctica. NewZealand Journal of Geology and Geophysics, 21, 686-698.
Hamilton, W. 1972. The Hallett Volcanic province, Antarctica (U.S. Geo-logical Survey Professional Paper 456-C). Washington, D.C.: U.S.Government Printing Office.
Kreuzer, H., A. HOhndorf, H. Lenz, U. Vetter, F. Tessensohn, P. Muller,H. Jordon, W. Harre, and C. Besang. 1981. K/Ar and Rb/Sr dating ofigneous rocks from northern Victoria Land, Antarctica. GeologischesJahrbuch, B41, 267-273.
Nathan, S., and E J. Schulte. 1967. Recent thermal and volcanic activity
on Mt. Melbourne, northern Victoria Land, Antarctica. New ZealandJournal of Geology and Geophysics, 10, 422-430.
Nathan, S., and F. J. Schulte. 1968. Geology and petrology of the Camp-bell-Aviator divide, northern Victoria Land, Antarctica: Part 1. Post-Paleozoic rocks. New Zealand Journal of Geology and Geophysics, 11,940-975.
Riddols, B. W., and C. T. Hancox. 1968. Geology of the upper MarinerGlacier region, north Victoria Land, Antarctica. New Zealand Journal ofGeology and Geophysics, 11, 881-899.
Stump, E., J. R. Holloway, S. G. Borg, and K. F. Lapham. 1982. Geo-logical investigations on early to middle Paleozoic magmatic rocks,northern Victoria Land, Antarctica. Antarctic Journal of the U.S., 17(5),17-18.
Paleontology of the Lower Paleozoicof northern Victoria Land:
Brachiopods with Australian and NewZealand affinities in the Spurs
Formation
A. J. ROWELL
Department of Geologyand
Museum of Invertebrate PaleontologyUniversity of Kansas
Lawrence, Kansas 66045
R. A. COOPER
New Zealand Geological SurveyLower Hutt, New Zealand
J. B. JAGO
South Australia Institute of TechnologyIngle Farm, South Australia
P. BRADDOCK
Picton, New Zealand
During the austral summer of 1981-1982, we investigated thepaleontology of the Bowers Supergroup of northern VictoriaLand (figure 1) as event K20 of the New Zealand AntarcticResearch Program. Although it was under New Zealand spon-sorship and had two New Zealand members, the team had aninternational composition because it also included an Aus-tralian and an American. Much of the season we worked inclose cooperation with members of event K19 (M.G. Laird, J . D.Bradshaw, C. J. D. Adams, and K. Sullivan) who were studyingthe sedimentology and physical stratigraphy of the same se-quence of Lower Paleozoic rocks.
Transportation was primarily by motor toboggan-drawnsledge and during the period 17 November 1981 through 3January 1982 we covered some 1,200 kilometers. Our fieldworkwas aided substantially by the availability of helicopter supportfor establishing food and fuel depots and for geological recon-naissance of outlying areas.
Our principal objective was to make fossil collections thatwould better constrain the age of the Bowers Supergroup andits various formations. In this we were successful and shortaccounts are available (Cooper, Jago, and Rowell 1982b; inpress). In outline, it is now apparent that the oldest beds of thesupergroup are much younger than had previously beenthought. The base of the Sledgers Group (figure 1) is unlikely tobe older than Middle Cambrian. Earlier views regarded thesebasal beds of the "Bowers Trough" as Vendian (Cooper et al.1982a; Laird 1981; Laird, Bradshaw, and Wodzicki 1982; Tessen-sohn et al. 1981), an age based upon acritarchs that are nowknown to range into post-Vendian rocks (Vidal in Cooper et al.in press). This revised date for the initiation of deposition in thetrough has considerable paleogeographic implication. Further-more, it alters our perception of the magnitude of the strati-
MIDDLE CAMBRIAN I UPPER CAMBRIAN LORD.
SLEDGERSMARINERLEAP YEARGROUPGROUP GROUP
o0M
3o3Ct)C CM C)
o0 I N
3
CD
tic)0
C)11 UH
0 3Cn
Figure 1. Stratigraphic subdivision and age of the Bowers Super-group (after Cooper et al. in press).
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