The recent suggestion that the Sudbury Igneous Complex (SIC) is a fractionated impact melt may have profound implications for understanding the lunar crust and the magmatic history of the Moon. A cornerstone of much current thought on the Moon is that the development of the lunar crust can be traced through the lineage of 'pristine' igneous rocks. However, if rocks closely resembling those from layered igneous intrusions can be produced by differentiation of a large impact melt sheet, then much of what is thought to be known about the Moon may be called into question. This paper presents a brief evaluation of the SIC as a differentiated impact melt vs. endogenous igneous magma and possible implications for the magmatic history of the lunar crust

Norman, Marc D.

English

NASA Technical Reports Server

54 7992 Sudbury Conference
Application of principles of hypervelocity impact entering has
provided overwhelming evidence for an impact origin of the Indian
crater. Among others, shock metantorphic characteristics of basalt,
impact glasses, mineralogy, chemistry, geochemistry, and compari-
son with the Moon's rocks have clearly demonstrated its formation
by impact of a meteorite [3-6].
Over the yean, the origin of the Lonar structure has risen from
volcanism, subsidence, and cryplovolcanism to an authentic mete-
orite impact crater. Lonar is unique because it is probably the only
terrestrial crater in basalt and is the closest analog with the Moon's
craters. Some unresolved questions are suggested. The proposal
is made that the young Lonar impact crater, which is less than
50.000 years old, should be considered as the best crater laboratory
analogous 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 a
fundamental ubiquitous geological process in the evolution of the
planets [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. Earth
Sci.. 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 OR
IGNEOUS ROCK? IMPLICATIONS FOR LUNAR MAG-
MATISM. Marc D.Norman, Planetary Geosciences, Department
of 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 for
understanding the lunar crust and the magmatic history of the Moon.
A cornerstone of much current thought on the Moon is that the
development of the lunar crust can be traced through the lineage of
"pristine" igneous rocks [2]. However, if rocks closely resembling
those from layered igneous intrusions can be produced by differen-
tiation of a large impact melt sheet, then much of what is thought to
be known about the Moon may be called into question. This paper
presents a brief evaluation of the SIC as a differentiated impact melt
vs. endogenous igneous magma and possible implications for the
magmatic history of the lunar crust
Petrologic and geochemical studies of terrestrial impact melts
have shown that most of these occurrences cooled quickly, creating
homogeneous crystalline rocks with compositions approximating
those of the average target stratigraphy [3,4]. Impact melts typi-
cally, but not always, have elevated concentrations of siderophile
elements relative to the country rock, indicating meteoritic con-
tamination [5,6,7]. Application of these studies to lunar samples has
lead to various criteria thought to be useful for distinguishing
primary igneous rocks of the lunar highlands crust from the mix-
tures created by impact melting [2.8,9]. Among these criteria are
mineral compositions suggesting plutonic conditions, non-KREEPy
incompatible trace-element patterns, and low concentrations of
meteoritic siderophile elements. Lunar breccias and impact melts
identified as poly mict on petrographic grounds usually have incom-
patible- and siderophile-element signatures indicating K KEEP and
meteoritic components, so a lack of these components may be taken
as evidence that a sample preserves its primary igneous composi-
tion, even though its texture may have been modified by cataclasis
or annealing.
If the SIC represents melt formed during the impact event that
created the Sudbury Basin, then ideas of how large melt sheets
behave require revision. The SIC is anori tic-to-gran ophyric mass of
crystallized silicate liquid with mineral and chemical compositions
broadly consistent with closed-system fractional crystallization,
although greenschist facies alteration has obscured much of the
fine-scale record [10]. Despite the Ni and PGE (platinum-group-
element) sulfide ores in the SIC. siderophile-element abundances in
the silicates are comparable to those of the country rock [11]. PGE
patterns in the ores are not chondritic, as they are in many impact
melts, but are highly fractionated and similar to those of terrestrial
basalts [12]. Osmium isotopic compositions in the ores suggest a
significant component of continental crust and are difficult to
reconcile 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 can
create melt rocks with characteristics indistinguishable from those
of layered igneous intrusions and with no detectable meteoritic
contamination, then any or all of the pristine lunar highland rocks
may not necessarily represent endogenous lunar magmatism but
fractionated impact melts. Diverse components would still be
required in the lunar crust and/or upper mantle to produce the
impressive array of lunar highland rock types, but the connection to
major mantle reservoirs that could constrain the planet's bulk
composition would be lost There may be economic implications as
well: If the SIC is a fractionated impact melt, then large impact
structures become potential exploration targets, both terrestrial and
extraterrestrial.
Despite the somewhat unusual, silica-rich bulk composition of
the SIC, several characteristics of the complex appear more consis-
tent with endogenous magmatic processes vs. impact melting and in
situ differentiation. Among these characteristics are (1) petrologic
and geophysical evidence suggesting an unexposed mafic or ultra-
mafic mass beneath the SIC, (2) contact relations within the SIC that
suggest multiple intrusive events, and (3) possibly abundant water
in the SIC magma. In addition, we argue that the silica-rich
composition of the SIC does not require impact melting, but can be
accounted for by endogenous magmatic processes although the
impact event may have influenced the course of the magmatic
evolution. These topics are discussed in more detail below.
Petrologic and geophysical evidence favoring an unexposed
mafic or ultramafic mass beneath the SIC includes ultramafic
xenoliths found in the SIC sublayer [13,14] and gravity and mag-
netic data [15]. The xenoliths have mineral and trace-element
compositions suggesting a petrogenetic connection to the SIC. A
basal ultramafic zone would suggest that the SIC is not contained
entirely within the impact structure and would create a bulk compo-
sition for the SIC unlike that of the proposed target stratigraphy. It
would also seem to require a mantle-derived component in the SIC
magma, which may be more supportive of an endogenous magmatic
origin rather than incorporation of mantle material into the impact
melt. If the excavation cavity of the Sudbury impact event was
100-150 km diameter [1], the depth of excavation was probably
<20 km [ 16], which would be predominantly or exclusively within
the continental crust.
https://ntrs.nasa.gov/search.jsp?R=19930000981 2020-03-17T10:04:29+00:00Z
LPI Contribution No. 790 55
Contact relations between the sublayer and the SIC main mass
norite appear to reflect multiple intrusive events although both units
may have been mobile simultaneously [14]. Multiple intrusions
would seem more consistent with pulses of endogenous magmatism
rather than a one-shot impact event although the mechanics of large-
scale impact melting remain obscure. Amphibole is present in the
SIC norite and may be primary [10]. The presence of water in the
melt in amounts necessary to stabilize amphibole (2-5 wt%) may be
more consistent with an endogenous magma rather than a super-
heated impact melt. For example, tektites are among the driest of
terrestrial rocks, but their small volume may not be directly analo-
gous to the SIC. It may also be possible that a dry impact melt
became hydrated through assimilation of country rock during
crystallization.
The bulk composition of the SIC seems to be close to mat of an
average for the upper crustal target stratigraphy [1], which is a
common characteristic of terrestrial impact melts. However, endo-
genous magmatic processes such as assimilation can incorporate
significant amounts of continental crust into more mafic magmas
without superheat [17,18]. Such processes can produce igneous
rocks with compositional characteristics quite similar to that esti-
mated for the bulk composition of the SIC. For example, many
occurrences of Cenozoic volcanic rocks in western North America
have bulk compositions close to that of the SIC [19-22].
Even if the SIC is not a direct impact melt, there does appear to
be a close association in space and time between the SIC and a major
impact event Dietz [23] and French [24] described features in the
Sudbury Basin that they attributed to shock. Their arguments that
the Basin is an impact structure are persuasive because there are no
known occurrences of similar shock features unequivocally associ-
ated with volcanic eruptions. If the Sudbury Basin is an impact
structure, it is the largest such structure known on Earth. The
noncirculari ty of the SB has been cited as evidence against an impact
origin, although the original shape of the Basin is poorly constrained
[25]. Although die original shapes of most impact craters generally
are circular, considerable variation in crater outline and morphol-
ogy can be found. Oblique impacts can produce craters with
elongate outlines, as observed on the Moon and Mars [26-29]. An
oblique, skipping impact event that created a series of elongated
scars was discovered recently in Peru [30]. Fragmentation of the
impactor can produce elongated, noncircular crater patterns or
multiple events as shown by the Henbury cluster, the Cape York
meteorite field, and the East-West Clearwater pair. Erosion and
deformation can alter the original shape of an impact basin, e.g.,
MeteorCrater is somewhat rectangular. The apparent noncircularity
of the Sudbury crater is not a strong argument against an impact
origin when stacked against the host of shock features clearly
associated with the Basin.
Even if the SIC is an endogenously produced magma and not an
impact melt, the association of impact events and magmatism may
nonetheless have important implications when considering the
locus and style of planetary magmatism. The close correspondence
in space and time between the impact event and the magmatism that
produced the SIC suggests a broadly genetic connection, especial-
ly considering the overall paucity of magmatism of similar age
(18SO Ma) in the region [31,32]. In order to explain the composi-
tional characteristics of the SIC, it appears necessary to invoke
significant mixing of mantle-derived magmas with continental
crust. Spatial variations in mineral compositions away from wall
rock contacts suggest that the melt was actively assimilating wall
rock [ 10]. In tracra ter melt rocks or breccias may have been assimi-
latcd by more mafic magmas, which in turn may have been produced
by local thermal perturbations or pressure-release melting associ-
ated with the impact.
Alternatively, crustal material may have been injected into die
mantle, producing a mixed source that melted to give the SIC parent
magma. Nyquist and Shih [33] have proposed that regional hetero-
geneities in die lunar mantle may reflect large impact events that
injected crustal material deep into the Moon's interior.
The SIC appears to represent endogenous magmatism although
probably localized and influenced by a major impact event and
structure. The role of pristine lunar highland rocks as products of
endogenous magmatism is correspondingly secure for die moment
but the effects of major impact events in localizing and influencing
that magmatism remains poorly perceived and probably requires
additional missions to die Moon to clarify. Regardless, study of
impact events remains of fundamental importance for understand-
ing die formation and evolution of die planets.
References: [l]Grieveetal.(1991)./G/?,96.22753. [2] War-
ren and Wasson(1977)/>roc.ISCS/A,2215;(1978)/Voc.L/>SC9rti>
185. [3] Phinney and Simonds (1977) Impact and Explosion
Cratering, 771. [4] Grieve et al. (1977) Impact and Explosion
Cratering. 791. [5] Morgan et al. (1975) Proc. LSC 6th. 1609.
[6] Palme et al. (1978) CCA, 42.313. [7] Wolf et al. (1980) GCA.
44. 1015. [8] Warner and Bickle (1978) Am. Mineral.. 63. 1010.
[9] Ryder et al. (1980) Proc. LPSC llth. 471. [10] Naldrett and
Hewins (1984) Ontario Geol. Surv. Spec. Vol. 1.235. [11] Walker
et al. (1991) EPSL, 105.416. [12] Naldrett (1984) Ontario Geol.
Surv. Spec. Vol. 1,309. [13] Scribbins et al. (1984) Can. Mineral..
22,67. [14] Naldrett et al. (1984) Ontario Geol. Surv. Spec. Vol. 1,
253. [15] Gupta el d.(l9M) Ontario Geol. Surv. Spec. Vol.1, 381.
[16] Pike and Spudis (1987) Earth Moon Planets, 39, 129.
[17] Leeman and Hawkesworth (1986) JGR, 91.5901. [18] Moor-
bath and Hildreth (1988) CMP. 98,455. [19] Gerlach and Grove
(1982) CMP, 80.147. [20] McMillan and Duncan (1988) J. Petrol..
29. 527. [21] Nixon (1988) J. Petrol.. 29. 265. [22] Norman and
Mertunan (1991) JGR .96. 13279. [23] Dietz (1964) J. Geol.. 72.
412. [24] French (1970) Bull. Volcanol.. 34.466. [25] Shanks and
Schwerdtner (1991) Can. J. Earth. Sci.. 28, 411. [26] Wilhelms
(1987) US. Geol. Surv. Prof. Pap. 1348. [27] Schultz and Lutz-
Garihan (1982) Proc. LPSC 13th, 84. [28] Nyquist (1983) Proc.
LPSC 13th, 785; (1984) Proc. LPSC 14th, 631. [29] Mouginis-Mark
et al. (\992)JGR, in press. [30] Schultz and Lianza (1992) Nature,
355.234. [31] Sims et al. (1981) Geol. Surv. Can. Pap. 81-10,379.
[32] Krogh et al. (1984) Ontario Geol. Surv. Spec. Vol. 1. 431.
[33] Nyquist L. E. and Shih C.-Y. (\992) LPSCXXIII. 1007-1008.
MELTING ANDITS RELATIONSHIPTOIMPACTCRATER
MORPHOLOGY. John D. O'Keefe and Thomas J. Ahrens,
LJndhurst Laboratory of Experimental Geophysics, Seismological
Laboratory 252-21, California Institute of Technology, Pasadena
CA 91125. USA.
Shock-melting features occur on planets at scales that range
from micrometers to megameters. It is the objective of this study to
determine die extent of thickness, volume geometry of the melt, and
relationship with crater morphology.
The variation in impact crater morphology on planets is influ-
enced by a broad range of parameters: e.g., planetary density (p).
thermal state, strength (Y), impact velocity (U), gravitational
acceleration (g).... We modeled die normal impact of spherical


Sudbury Igneous Complex: Impact melt or igneous rock? Implications for lunar magmatism

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