LPI Contribution No. 790 S3

[7], who found that in order to generate a ring fault at a distance of-1 .4 crater radii, it was necessary to restrict aslhenospheric flow toa channel at depth, one overlying a stiff er mesosphere. It is temptingto assign this asthenospheric channel to a ductile lower crust, asdiscussed above. Alternatively, an effectively stiffer mesospheremay be a natural consequence of truly non-Newtonian rebound.Much work remains to be done on mis problem.

Overall, these estimates and models suggest that multiringedbasin formation is indeed possible at the scales observed on Venus.Furthermore, due to the strong inverse dependence of solid-stateviscosity on stress, the absence of Cordilleran-style ring faulting incraters smaller than Meitner or Kknova makes sense. The (1) ap-parent increase in viscosity of shock-fluidized rock with craterdiameter. (2) greater interior temperatures accessed by larger.deeper craters, and (3) decreased non-Newtonian viscosity associ-ated with larger craters may conspire to make the transition withdiameter from peak-ring crater to Orientalc- type multiringed basinrather abrupt.

References: [1] Alexopoulos J. S. and McKinnon W. B.(1992)JGR, submitted. [2] GaultD. E . and SonettC. P. (1982)G&4Spec. Pap., 190. 69-102. [3] Melosh H. I. and Gaffney E. S. (1983)Proc. LPSC 13th. in JGR. 88. A830-A834. [4] Cintala M. J. andGrieve R. A. F. (1991) LPSC XX II, 213-214. [S] Campbell D. B. etal. (1991) JGR, in press. [6] Melosh H. J. and McKinnon W. B.(1978) GRL. 5. 985-988. [7] Melosh H. J. and Hillgren V.J. (1987)LPSC XVIII, 639-640.

SUDBURY PROJECT (UNIVERSITY OF MONSTER-ONTARIO GEOLOGICAL SURVEY): (5) NEW INVESTI-GATIONS ON SUDBURY BRECCIA. V.Mfiller-Mohr, Insti-tute of Planetology, University of MOnster. Wilhelm-Klemm-Str.10, W-4400 Munster, Germany. kA ___ ff^./ (J

Sudbury breccias occur as discordant dike breccias within thefbotwall rocks of the Sudbury structure, which is regarded as thepossible remnant of a mul tiring basin [1]. Exposures of Sudburybreccias in the North Range are known up to a radial distance of60-80 km from the Sudbury Igneous Complex (SIC). The brecciasappear more frequent within a zone of 1 0 km adjacent to the SIC anda further zone located about 20-33 km north of the structure.

From differences in the structure of the breccias, as for examplethe size of the breccia dikes, contact relationships between brecciaand country rock as well as between different breccia dikes, frag-ment content, and fabric of the ground mass, as seen in thin section,the Sudbury Breccias have been classified into four different types.

A. Early breccias with a clastic/crystalline matrix comprisesmall dikes ranging in size from -1 cm to max. 20cm. Characteristicfeatures of these breccias are sharp contacts to country rock, lowfragment content (20-30%), local origin of fragments, and anaphanitic, homogenous matrix, which can be related to countryrock. Locally corrosional contacts to feldspar minerals and smallvesicles filled with secondary minerals are observed.

B. Polymict breccias with a clastic matrix represent the mostcommon type of Sudbury breccia. The thickness of the dikes variesfrom several tens of centimeters to a few meters but can also extendto more than 100 m in the case of the largest known breccia dike.Contacts with country rock are sharp or gradadonal. Fragmentcontent (60-75%) is usually of local origin but especially in largedikes allochthonous fragments have been observed. Inclusions oftype A breccias reveal the later formation of this type of breccia. The

heterogenous matrix consisting of a fine-grained rock flour displaysnonoriented textures as well as extreme flow lines. Chemicalanalysis substantiates at feast some mixing with allochthonousmaterial.

C. Breccias with a crystalline matrix are a subordinate type ofSudbury breccia. According to petrographical and chemical differ-ences, three subtypes have been separated. The local origin of thefragments and the close chemical relationship to the country rockpoint to an autochthonous generation probably through in situfrictional processes. For two subtypes the geometry of the dikes andthe texture of the matrix indicates that at feast some transport ofbreccia material has occurred. Breccias with a crystalline matrixhave never been observed in contact with the other types of breccias.

D. Late breccias with a clastic matrix are believed to representthe latest phase of brccciation. Two subtypes have been distin-guished due to differences in the fragment content. Breccias with alow fragment content show a weak lamination and sharp or grada-tional contacts to country rock. Inclusions of type A breccias areobserved. Breccias with a high fragment content are characterizedby gradational contacts and are only known from the outermost partsof the structure. Fragments of these breccias are of local origin. Apossible correlation of the relative timescale of breccia formationwith the phases of crater formation will be discussed.

Shock deformation features, which have been recorded withinbreccia fragments up to a radial distance of 9 km from the SIC,represent the shock stage I of the basement rocks. Inclusionsexhibiting a higher shock stage, such as melt particles or sueviticfragments, which are known from dike breccias of, e.g., the Cars wellimpact structure, are lacking. This means that the dike breccias ofSudbury as presently exposed are from a deeper level of thesubcrater basement than their counterparts of Cars well.

References: [1] Stdffler et al. (1989) Meteoritics, 24, 328.

TI

A HISTORY OF THE LONAR CRATER, INDIA—ANOVERVIEW. V. K. Nayak. Department of Applied Geology.Indian School of Mines, Dhanbad, India.

The origin of the circular structure at Lonar, India(19°581N:76°3rE). described variously as cauldron, pit, hollow,depression, and crater, has been a controversial subject since theearly nineteenth century. A history of its origin and other aspectsfrom 1823 to 1990 are overviewed. The structure in the Deccan TrapBasalt is nearly circular with a breach in the northeast, 1830 m indiameter, ISO m deep, with a saline lake in the crater floor.

Since time immemorial, mythological stories prevailed to ex-plain in some way the formation of the Lonar structure, which hasbeen held in great veneration with several temples within andoutside the depression. Various hypotheses proposed to understandits origin are critically examined and grouped into four categoriesas (1) volcanic, (2) subsidence, (3) cryptovolcanic, and (4) meteor-ite impact. In the past, interpretations based on geological, morpho-logical, and structural data were rather subjective and dominated byvolcanic, subsidence, and, to some extent, cryptovolcanic explana-tions [ 1 ]. In 1960, experience of the Canadian craters fed Beals et al.[2] to first suggest the possibility of a meteorite impact origin of theLonar crater, and thus began a new era of meteorite impact in thehistory of the Indian crater.

The last three decades (1960 to 1990) reflect a period of greatexcitement and activity of the Lonar crater, perhaps owing to anupsurge of interest in exploration of the Moon and other planets.

54 7992 Sudbury Conference

Application of principles of hypervelocity impact entering hasprovided overwhelming evidence for an impact origin of the Indiancrater. Among others, shock metantorphic characteristics of basalt,impact glasses, mineralogy, chemistry, geochemistry, and compari-son with the Moon's rocks have clearly demonstrated its formationby impact of a meteorite [3-6].

Over the yean, the origin of the Lonar structure has risen fromvolcanism, subsidence, and cryplovolcanism to an authentic mete-orite impact crater. Lonar is unique because it is probably the onlyterrestrial crater in basalt and is the closest analog with the Moon'scraters. Some unresolved questions are suggested. The proposalis made that the young Lonar impact crater, which is less than50.000 years old, should be considered as the best crater laboratoryanalogous to those of the Moon, be treated as a global monument,and preserved for scientists to comprehend more about the myster-ies of nature and impact entering, which is now emerging as afundamental ubiquitous geological process in the evolution of theplanets [7].

References: [1] La louche T. H. D. and Christie W. A. K.(1912) Rec. Geol Surv. India, 41. 266-285. [2] Beals C. S. et al.(1960) Curr<fn/Sc»«ic<./mfia, 29. 205-218. [3] NayakV.K. (1972)EPSL. 14. 1-6. [4] Fredriksson K. et al. (1973) Science, 180.862-864. [5] Fredriksson K. et al. (1979) Smiihson. Contrib. EarthSci.. 22. 1-13. [6] Kieffer S. W. et al. (1976) Proc. LSC 7th,1391-1412. [7] Grieve R. A. F. and Head J. W. (1981) Episodes, 4.3-9-

SUDBURY IGNEOUS COMPLEX: IMPACT MELT ORIGNEOUS ROCK? IMPLICATIONS FOR LUNAR MAG-MATISM. Marc D.Norman, Planetary Geosciences, Departmentof Geology and Geophysics, SOEST, University of Hawaii, Honolulu

. USA.

The recent suggestion that the Sudbury Igneous Complex (SIC)is a fractionated impact melt [ 1 J may have profound implications forunderstanding the lunar crust and the magmatic history of the Moon.A cornerstone of much current thought on the Moon is that thedevelopment of the lunar crust can be traced through the lineage of"pristine" igneous rocks [2]. However, if rocks closely resemblingthose from layered igneous intrusions can be produced by differen-tiation of a large impact melt sheet, then much of what is thought tobe known about the Moon may be called into question. This paperpresents a brief evaluation of the SIC as a differentiated impact meltvs. endogenous igneous magma and possible implications for themagmatic history of the lunar crust

Petrologic and geochemical studies of terrestrial impact meltshave shown that most of these occurrences cooled quickly, creatinghomogeneous crystalline rocks with compositions approximatingthose of the average target stratigraphy [3,4]. Impact melts typi-cally, but not always, have elevated concentrations of siderophileelements relative to the country rock, indicating meteoritic con-tamination [5,6,7]. Application of these studies to lunar samples haslead to various criteria thought to be useful for distinguishingprimary igneous rocks of the lunar highlands crust from the mix-tures created by impact melting [2.8,9]. Among these criteria aremineral compositions suggesting plutonic conditions, non-KREEPyincompatible trace-element patterns, and low concentrations ofmeteoritic siderophile elements. Lunar breccias and impact meltsidentified as poly mict on petrographic grounds usually have incom-

patible- and siderophile-element signatures indicating K KEEP andmeteoritic components, so a lack of these components may be takenas evidence that a sample preserves its primary igneous composi-tion, even though its texture may have been modified by cataclasisor annealing.

If the SIC represents melt formed during the impact event thatcreated the Sudbury Basin, then ideas of how large melt sheetsbehave require revision. The SIC is anori tic-to-gran ophyric mass ofcrystallized silicate liquid with mineral and chemical compositionsbroadly consistent with closed-system fractional crystallization,although greenschist facies alteration has obscured much of thefine-scale record [10]. Despite the Ni and PGE (platinum-group-element) sulfide ores in the SIC. siderophile-element abundances inthe silicates are comparable to those of the country rock [11]. PGEpatterns in the ores are not chondritic, as they are in many impactmelts, but are highly fractionated and similar to those of terrestrialbasalts [12]. Osmium isotopic compositions in the ores suggest asignificant component of continental crust and are difficult toreconcile with meteoritic contamination [11].

A lunar sample with mineralogic and geochemical characteris-tics analogous to those of the SIC probably would be judged as"pristine," hence a primary igneous rock. If large impact events cancreate melt rocks with characteristics indistinguishable from thoseof layered igneous intrusions and with no detectable meteoriticcontamination, then any or all of the pristine lunar highland rocksmay not necessarily represent endogenous lunar magmatism butfractionated impact melts. Diverse components would still berequired in the lunar crust and/or upper mantle to produce theimpressive array of lunar highland rock types, but the connection tomajor mantle reservoirs that could constrain the planet's bulkcomposition would be lost There may be economic implications aswell: If the SIC is a fractionated impact melt, then large impactstructures become potential exploration targets, both terrestrial andextraterrestrial.

Despite the somewhat unusual, silica-rich bulk composition ofthe SIC, several characteristics of the complex appear more consis-tent with endogenous magmatic processes vs. impact melting and insitu differentiation. Among these characteristics are (1) petrologicand geophysical evidence suggesting an unexposed mafic or ultra-mafic mass beneath the SIC, (2) contact relations within the SIC thatsuggest multiple intrusive events, and (3) possibly abundant waterin the SIC magma. In addition, we argue that the silica-richcomposition of the SIC does not require impact melting, but can beaccounted for by endogenous magmatic processes although theimpact event may have influenced the course of the magmaticevolution. These topics are discussed in more detail below.

Petrologic and geophysical evidence favoring an unexposedmafic or ultramafic mass beneath the SIC includes ultramaficxenoliths found in the SIC sublayer [13,14] and gravity and mag-netic data [15]. The xenoliths have mineral and trace-elementcompositions suggesting a petrogenetic connection to the SIC. Abasal ultramafic zone would suggest that the SIC is not containedentirely within the impact structure and would create a bulk compo-sition for the SIC unlike that of the proposed target stratigraphy. Itwould also seem to require a mantle-derived component in the SICmagma, which may be more supportive of an endogenous magmaticorigin rather than incorporation of mantle material into the impactmelt. If the excavation cavity of the Sudbury impact event was100-150 km diameter [1], the depth of excavation was probably<20 km [ 16], which would be predominantly or exclusively withinthe continental crust.