Plutonic rocks of the Lassiter Coast
for 4n extended period of time, beginning with a singlemajor period of folding sometime after deposition lof Middle and Upper Jurassic sedimentary and volc.nic rocks and continuing until after middle Creticeous plutons were emplaced. Tliis study was supported by National Science Fourdation grant AG-187.
References Kellogg, K. S., and R. L. Reynolds. 1974. Paleomagnetic study of igneous rocks of the northern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(2) 38-40. Lau on, T. S. 1972. Stratigraphy of eastern Ellsworth Land. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, U iversitetsforlaget, Series B(1) 215-223.
Plutonic rocks of the Lassiter Coast PETER D. ROWLEY and PAUL L. WILLIAMS
U.S. Geological Survey Denver, Colorado 80225
Plutonic rocks underlie over a third of the 30,000square-kilometer Lassiter Coast area and were mapped by the U.S. Geological Survey (USGS) during the 1969-1970, 1970-1971, and 1972-1973 field seasons. Evaluation of petrographic, chemical, and field data indicates that these rocks belong to a single caic-alkaline igneous complex that is correlated with the Andean intrusive suite exposed elsewhere along the Antarctic Peninsula. The Lassiter Coast igneous complex intrudes tightly folded shale, siltstone, and sandstone of Middle and Late Jurassic age, as well as overlying ash flow tuff and andesitic lava flows of presumed Late Jurassic age (Williams et al., 1972). Some plutons reflect multiple intrusive events, and crosscutting relations prove that emplacement occurred at different times. Potassium-argon age determinations on biotite and hornblende from seven plutons in the southern and central Lassiter Coast range from 95 to 119 million years (A. H. Clark et al., personal communication; Mehnert et al., in press). The more felsic bodies generally are younger. Emplacement of each pluton was followed by intrusion of aplite and pegmatite and, in turn, by intrusion of mostly intermediate to mafic dikes. Hydrothermal alteration and copper mineralization occurred near the conclusion of igneous activity in some plutons (Rowley et al., in preparation). There are more than 50 known stocks and batholiths in which plutonic rocks occur. Most of the September-October 1974
Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. In press. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research. Plummer, C. C. 1974. Contact metamorphism of the Latady Formation, southern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(3) : 82-83. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., VIII (4) : 154-155. Rowley, P. D., P. L. Williams, D. L. Schmidt, A. B. Ford, R. L. Reynolds, A. H. Clark, E. Farrar, and S. L. McBride. In preparation. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Williams, P. L., and P. D. Rowley. 1971. Geologic studies of the Lassiter Coast. Antarctic Journal of the U.S., VI(4): 120. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula: preliminary report: In: Antarctic Geology and Geophysics. (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget, Series B(1): 143-148.
intrusions are oval shaped in plan with long axes commonly oriented in a northerly direction. The plutons range in size from less than 1 kilometer to a nearly continuous 20 kilometers wide strip extending from the central Lassiter Coast north about 150 kilometers into the Black Coast. In the southern third of the area most bodies are stocks with lengths
10
35
65
Ploqioclase K-feldspar (Including pert hite)
Figure 1. Plutonic rock classification triangle with long axes of fields of modal quartz and feldspar for those plutons in the Lassiter Coast defined by five or more modal analyses. Plots of rocks from associated aplite, pegmatite, or other dikes are not shown. This figure represents 221 modal analyses. Solid lines are plutons of northern Lassiter Coast (generally north of Swann Glacier); dashed lines are plutons of central Lassiter Coast; dotted lines are plutons of southern Lassiter Coast (south of Wetmore Glacier).
225
grained hypidiomorphic-granular texture with sme large (up to several centimeters) scattered poik1itic potassium-feldspar crystals. Potassium-feldspar r4rely is grid twinned or perthitic except in the fort em plutons; in all plutons most plagioclase is zo ed. Biotite and green hornblende are the common f rromagnesian minerals, but orthopyroxene and c mopyroxene are abundant in mafic plutons. Op que minerals consist of magnetite with less corn on ilmenite and pyrite. Sphene and apatite are lo ally abundant accessory minerals.
Ab+n
Or
Figure 2. Long axes of fields of normative quartz and feldspar for those plutons of the Lassiter Coast defined by three or more chemical analyses. Plots of rocks from associated aplite, pegmatite, or other dikes are not shown. This figure represents 85 chemical analyses of Lassiter Coast rocks. Narrow line symbols are from fig. 1. In addition, heavy line "a" represents analyses from Adie (1955, table Vila) and heavy line "b" represents analyses from Laudon (1972, table 3).
of as much as about 15 kilometers. In the northern part of the area batholiths predominate, and plutonic rock is significantly more abundant. Some exposed plutons probably are connected at depths; compositional differences, however, indicate that most represent discrete magma injections. Intrusive contacts are sharp with abundant apophyses extending into the wall rock. Intrusion was passive in part but mostly forcible. Rare stope blocks up to 30 meters long occur within several plutons, and older rocks and structures have been sharply tilted adjacent to most contacts. Fine grained chilled margins are characteristic of most stocks and batholiths. Mafic inclusions are abundant especially near intrusive contacts. Metamorphic aureoles rarely exceed andalusite hornfels grade (Plummer, 1974). Figs. 1 and 2 portray compositional fields of some of the plutons in the Lassiter Coast area. Most bodies display a considerable range in composition; although some are irregularly zoned, commonly they are concentrically zoned toward a more felsic interior. Differences in composition are believed to be due to magmatic differentiation during crystallization. Geologic mapping indicates that the most abundant rock type probably lies close to the granodioritequartz rnonzonite boundary (fig. 1). The average composition generally becomes progressively more felsic to the north in the field area. Most plutonic rocks have a medium to coarse 226
Plutonic rocks of the Lassiter Coast have an a! lime index of about 61. When oxide values plotted on a silica variation diagram (Rowley, 1 unpublished data), trends are definite and exi able: as Si0 2 increases, Al,O,, CaO, MgO, and Fe decrease, while K20 increases and Na 20 is stant to slightly increasing. Plots of major o from Andean plutonic rocks of nearby eastern worth Land (Laudon, 1972, table 3) and of northern Antarctic Peninsula (Adie, 1955, 1 Vila) define similar alkali-lime indices (about and, on the silica variation diagram, fall near the curves of Lassiter Coast rocks. Triangular plot4, of normative quartz and feldspar calculated from tese same major oxide values also define similar cu'ves (fig. 2). This study was supported by National Science Foundation grant AG-187. References
Adie, R. J . 1955. The petrology of Graham Land. Part II, the Andean granite-gabbro intrusive suite. Falkland' Islands Dependencies Survey. Scientific report, 12. 39p. Laudon, T. S. 1972. Stratigraphy of eastern Ellsworth Land. In: Antarctic Geology and Geophysics (Adie, R. J., editor). International Union of Geological Sciences. Olso, Universitetsforlaget. Series B( 1): 215-223. Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. In press. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research. Plummer, C. C. 1974. Contact metamorphism of the Laady Formation, southern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(3): 82-83. Rowley, P. D., P. L. Williams, D. L. Schmidt, A. B. Ford, R. L. Reynolds, A. H. Clark, E. Farrar, and S. L. McBride. In preparation. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula: preliminary report. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget. Series B(1): 143-148.
ANTARCTIC JOURNAL
References Kellogg, K. S., and R. L. Reynolds. 1974. Paleomagnetic study of igneous rocks of the northern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(2) 38-40. Lau on, T. S. 1972. Stratigraphy of eastern Ellsworth Land. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, U iversitetsforlaget, Series B(1) 215-223.
Plutonic rocks of the Lassiter Coast PETER D. ROWLEY and PAUL L. WILLIAMS
U.S. Geological Survey Denver, Colorado 80225
Plutonic rocks underlie over a third of the 30,000square-kilometer Lassiter Coast area and were mapped by the U.S. Geological Survey (USGS) during the 1969-1970, 1970-1971, and 1972-1973 field seasons. Evaluation of petrographic, chemical, and field data indicates that these rocks belong to a single caic-alkaline igneous complex that is correlated with the Andean intrusive suite exposed elsewhere along the Antarctic Peninsula. The Lassiter Coast igneous complex intrudes tightly folded shale, siltstone, and sandstone of Middle and Late Jurassic age, as well as overlying ash flow tuff and andesitic lava flows of presumed Late Jurassic age (Williams et al., 1972). Some plutons reflect multiple intrusive events, and crosscutting relations prove that emplacement occurred at different times. Potassium-argon age determinations on biotite and hornblende from seven plutons in the southern and central Lassiter Coast range from 95 to 119 million years (A. H. Clark et al., personal communication; Mehnert et al., in press). The more felsic bodies generally are younger. Emplacement of each pluton was followed by intrusion of aplite and pegmatite and, in turn, by intrusion of mostly intermediate to mafic dikes. Hydrothermal alteration and copper mineralization occurred near the conclusion of igneous activity in some plutons (Rowley et al., in preparation). There are more than 50 known stocks and batholiths in which plutonic rocks occur. Most of the September-October 1974
Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. In press. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research. Plummer, C. C. 1974. Contact metamorphism of the Latady Formation, southern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(3) : 82-83. Rowley, P. D. 1973. Geologic observations on the northern Lassiter Coast and southern Black Coast. Antarctic Journal of the U.S., VIII (4) : 154-155. Rowley, P. D., P. L. Williams, D. L. Schmidt, A. B. Ford, R. L. Reynolds, A. H. Clark, E. Farrar, and S. L. McBride. In preparation. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Williams, P. L., and P. D. Rowley. 1971. Geologic studies of the Lassiter Coast. Antarctic Journal of the U.S., VI(4): 120. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula: preliminary report: In: Antarctic Geology and Geophysics. (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget, Series B(1): 143-148.
intrusions are oval shaped in plan with long axes commonly oriented in a northerly direction. The plutons range in size from less than 1 kilometer to a nearly continuous 20 kilometers wide strip extending from the central Lassiter Coast north about 150 kilometers into the Black Coast. In the southern third of the area most bodies are stocks with lengths
10
35
65
Ploqioclase K-feldspar (Including pert hite)
Figure 1. Plutonic rock classification triangle with long axes of fields of modal quartz and feldspar for those plutons in the Lassiter Coast defined by five or more modal analyses. Plots of rocks from associated aplite, pegmatite, or other dikes are not shown. This figure represents 221 modal analyses. Solid lines are plutons of northern Lassiter Coast (generally north of Swann Glacier); dashed lines are plutons of central Lassiter Coast; dotted lines are plutons of southern Lassiter Coast (south of Wetmore Glacier).
225
grained hypidiomorphic-granular texture with sme large (up to several centimeters) scattered poik1itic potassium-feldspar crystals. Potassium-feldspar r4rely is grid twinned or perthitic except in the fort em plutons; in all plutons most plagioclase is zo ed. Biotite and green hornblende are the common f rromagnesian minerals, but orthopyroxene and c mopyroxene are abundant in mafic plutons. Op que minerals consist of magnetite with less corn on ilmenite and pyrite. Sphene and apatite are lo ally abundant accessory minerals.
Ab+n
Or
Figure 2. Long axes of fields of normative quartz and feldspar for those plutons of the Lassiter Coast defined by three or more chemical analyses. Plots of rocks from associated aplite, pegmatite, or other dikes are not shown. This figure represents 85 chemical analyses of Lassiter Coast rocks. Narrow line symbols are from fig. 1. In addition, heavy line "a" represents analyses from Adie (1955, table Vila) and heavy line "b" represents analyses from Laudon (1972, table 3).
of as much as about 15 kilometers. In the northern part of the area batholiths predominate, and plutonic rock is significantly more abundant. Some exposed plutons probably are connected at depths; compositional differences, however, indicate that most represent discrete magma injections. Intrusive contacts are sharp with abundant apophyses extending into the wall rock. Intrusion was passive in part but mostly forcible. Rare stope blocks up to 30 meters long occur within several plutons, and older rocks and structures have been sharply tilted adjacent to most contacts. Fine grained chilled margins are characteristic of most stocks and batholiths. Mafic inclusions are abundant especially near intrusive contacts. Metamorphic aureoles rarely exceed andalusite hornfels grade (Plummer, 1974). Figs. 1 and 2 portray compositional fields of some of the plutons in the Lassiter Coast area. Most bodies display a considerable range in composition; although some are irregularly zoned, commonly they are concentrically zoned toward a more felsic interior. Differences in composition are believed to be due to magmatic differentiation during crystallization. Geologic mapping indicates that the most abundant rock type probably lies close to the granodioritequartz rnonzonite boundary (fig. 1). The average composition generally becomes progressively more felsic to the north in the field area. Most plutonic rocks have a medium to coarse 226
Plutonic rocks of the Lassiter Coast have an a! lime index of about 61. When oxide values plotted on a silica variation diagram (Rowley, 1 unpublished data), trends are definite and exi able: as Si0 2 increases, Al,O,, CaO, MgO, and Fe decrease, while K20 increases and Na 20 is stant to slightly increasing. Plots of major o from Andean plutonic rocks of nearby eastern worth Land (Laudon, 1972, table 3) and of northern Antarctic Peninsula (Adie, 1955, 1 Vila) define similar alkali-lime indices (about and, on the silica variation diagram, fall near the curves of Lassiter Coast rocks. Triangular plot4, of normative quartz and feldspar calculated from tese same major oxide values also define similar cu'ves (fig. 2). This study was supported by National Science Foundation grant AG-187. References
Adie, R. J . 1955. The petrology of Graham Land. Part II, the Andean granite-gabbro intrusive suite. Falkland' Islands Dependencies Survey. Scientific report, 12. 39p. Laudon, T. S. 1972. Stratigraphy of eastern Ellsworth Land. In: Antarctic Geology and Geophysics (Adie, R. J., editor). International Union of Geological Sciences. Olso, Universitetsforlaget. Series B( 1): 215-223. Mehnert, H. H., P. D. Rowley, and D. L. Schmidt. In press. K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica. USGS Journal of Research. Plummer, C. C. 1974. Contact metamorphism of the Laady Formation, southern Lassiter Coast, Antarctic Peninsula. Antarctic Journal of the U.S., IX(3): 82-83. Rowley, P. D., P. L. Williams, D. L. Schmidt, A. B. Ford, R. L. Reynolds, A. H. Clark, E. Farrar, and S. L. McBride. In preparation. Copper mineralization along the Lassiter Coast of the Antarctic Peninsula. Williams, P. L., D. L. Schmidt, C. C. Plummer, and L. E. Brown. 1972. Geology of the Lassiter Coast area, Antarctic Peninsula: preliminary report. In: Antarctic Geology and Geophysics (Adie, R. J . , editor). International Union of Geological Sciences. Oslo, Universitetsforlaget. Series B(1): 143-148.
ANTARCTIC JOURNAL
