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Macro-
and microscopic Evidence of impact Metamorphism in Rocks from
the Chicxulub peak ring
L. Ferri�re et al. -- Lunar and Planetary Science XLVIII
(2017); 1600.pdf
"The M0077A core was subdivided
into three main lithological units: a (post-impact) section
(from 505.7 to 617.3 mbsf), an upper (peak ring) section of
"suevite" and melt rocks (from 617.3 to 747.0 mbsf), and a
lower (peak ring) section mainly consisting of granitoid rocks
(with aplite and pegmatite dikes) intruded by different types
of subvolcanic dikes, and intercalations of millimeter to
decameter thick "suevite" and melt rock units (from
747.0 to 1334.7 mbsf).
The upper (peak ring) section consists of 104 m of "suevite"
(polymict lithic breccia with mm to over 25 cm in size melt
clasts and lithic mineral and rock fragments) on top of ~25
m melt rock (dominantly clast-poor but with clast-rich
intervals). The matrix of the "suevite" is calcitic (ranging
from micritic to sparitic). Clasts include a variety of more
or less shocked mineral and rock fragments (sedimentary [including
isolated fossils], metamorphic, and igneous lithologies, with
carbonate and granitoid being the most abundant rock types)
and melt fragments withaltered (green to brown in color in
plane-polarized-light; clay minerals) glassy to microcrystalline
textures. Many of the melt fragments have flow textures and
are occasionally vesicular. They often contain relic mineral
clasts (dominated by feldspars and quartz) and shocked lithic
rock fragments, or are, themselves, coated with an additional
layer of melt. Quartz grains show planar fractures (PFs) and/or
(decorated) planar deformation features (PDFs), with up to
3 sets. A few toasted quartz grains were noted. Silica glass,
generally recrystallized, with a chert-like appearance (and
with ballen silica also occur. Other minerals also exhibit
shock features, especially plagioclase and alkali-feldspar
(with PFs and PDFs). Possible coesite (??) was observed in
a large silica-rich melt fragment. The melt rock from the
lower part of the upper (peak ring) section is green to black
in color, with flow banding, and in some cases vesicular.
The green and black melts are locally intermixed, forming
schlieren of green material (altered melt?) in a black-colored
melt. A large variety of clasts are present (as in "suevite",
with the exception of sedimentary rock clasts that were not
found), in some cases so heavily shocked and/or hydrothermally
altered that it was difficult to identify them. Shock features
similar to those in "suevite" were observed in the melt rocks,
such as PFs and PDFs in quartz, toasted quartz, and a variety
of shock features in other minerals.
The lower (peak ring) section consists mainly of pervasively
deformed granitoid basement rocks (granite to syenite), overall
coarse-grained, with locally cm to dm thick aplitic and pegmatitic
sections. All main rock-forming minerals, i.e., alkali-feldspar,
plagioclase, quartz, and biotite, show signs of shock deformation.
In the case of quartz, locally PFs were even visible as a
result of preferential hydrothermal alteration. Almost all
quartz grains are shocked, with PFs, feather features (FFs),
and/or (decorated) PDFs ; up to 4 sets of PDFs are seen. Kinkbanding
was also observed for some quartz grains. Similar shock features
were observed in alkali-feldspar and plagioclase. Biotites
and chlorites are often kinked. Based on qualitative evaluation
of the thin sections no noticeable shock attenuation with
depth was observed. Preliminary quantitative results seem
to indicate little or no shock attenuation with depth.
For the first time shatter cones were
found at Chicxulub. They are well-developed in some of the
subvolcanic dikes intruding the granitoid basement rocks,
such as in aplite at 777.2 and 777.4 mbsf and in phonotephrite
dikes at 1125.1, 1131.7, 1137.5, and 1138.3 mbsf. A possible
poorly-developed shatter cone was also noted in a coarse-grained
granitoid sample at 909.6 mbsf. Finally, "suevites"and melt
rocks also occur in the lower (peak ring) section in the form
of small dikes and large bodies, such as the ~100 m thick
occurrence between 1215 and 1316 mbsf. Similar shock features
as in "suevites" and melt rocks from the upper (peak ring)
section were observed."
Emplacing impact melt in the Chicxulub peak
ring
David A. Kring et al. -- Lunar and Planetary
Science XLVIII (2017); 1213.pdf
"Distribution of Melt in the
Core: After penetrating (post-impact) sediments (Unit
1), the top of the (peak ring) was encountered at 617.33
mbsf, beginning with a 104-m-thick polymict, melt-bearing
breccia (Unit 2) with a calcitic
matrix that may represent a plume of carbonate ash. The most
abundant clasts in the breccia are melt fragments. That unit
has been sub-divided (2A, 2B, 2C) based on sedimentary and
matrix features. A melt rock, Unit 3,
extends ~26 m to a depth of 747 mbsf. It is dominantly a clast-poor
melt rock, but clast-rich intervals occur at ~722, ~732-734,
and ~744 mbsf. It has been subdivided to reflect a change
from green schlieren-bearing black melt (3A) to a basal ~9.5
m-thick coherent black melt unit (3B). Those units cover granite
and related basement lithologies. Thin, <1 m-thick melt horizons
were logged within the granite. A thicker (~4 m) series of
melt and melt-bearing breccia horizons were logged at ~1000
mbsf and ~58 m of melt and melt-bearing breccias dominate
the lower 100 m of core. The total thickness of the basement
interval sampled by the borehole is 588 m.
Clast Content: There is a diverse array of sedimentary,
metamorphic, and igneous clasts within those units. Sedimentary
lithologies are carbonate, chert that in many cases is visibly
associated with and derived from carbonate, shale, sandstone,
and red silt-stone. Metamorphic lithologies are gneiss, mylonite,
schist, amphibolite, and quartzite. Marble was also logged,
but thin-section studies are needed to determine if it is
a target unit or shock-modified carbonate. Igneous lithologies
include granite, granodiorite, diorite, dacite, felsite, and
mafic clasts that were variously logged as gabbro, diabase,
and dolerite. Carbonate and granite are the most abundant
lithologies.
Sedimentary, metamorphic, and igneous lithologies are found
in the uppermost units, but carbonate was not logged in the
bottom eight cores (from ~722 to ~747 mbsf) of the Unit 3
melt rock, and granite dominates the clast assemblage at the
base of that interval. Thin melt horizons within the granite
only have clasts of (locally derived) granite. However, the
horizon at ~1000 mbsf contains clasts of melt, granite, granodiorite,
dolerite, and gneiss. The basal interval of melt and melt-bearing
breccias also has a diverse array of metamorphic and igneous
clasts, but no sedimentary clasts.
Discussion: For the melt-bearing horizons within the
granite, we consider four hypotheses. (1) The "impact" melts
in the granite may have been injected into the walls of the
transient crater and transported with the bounding granite
during uplift and collapse into a peak ring. This seems unlikely,
however, because the granite is heavily sheared while the
impact melt rocks and melt-bearing breccias are not. The melts
were introduced after most (albeit not all) of the deformation
of the peak ring had occurred.
The "impact" melts in the granite may have instead been (2)
produced by melting along shear planes and faults in the basement.
That is a likely source of the thin, <1 m melt horizons, which
only contain clasts of granite. However, there is a wide variety
of clasts in the thicker melt-bearing units at ~1000 mbsf
and at the base of the borehole that would require transport
of multiple lithologies (granodiorite, gneiss, dolerite) an
unknown distance along an intrusive conduit and emplacement
in granite.
We cannot discount, however, (3) the infusion of melt from
adjacent melt pools along open fractures in the peak ring.
The melts at the base of the borehole lie at a greater depth
than "impact" melt in the adjacent crater trough, although
that source may require transport through fractures over a
distanceof at least 2 km, without quenching, while mixing
with clasts from multiple peak-ring lithologies.
An alternative (ludicrously) hypothesis
(4) is prompted by the lack of sedimentary clasts in the deeper
"impact" melts. If the dynamic collapse and Displaced Structure
Uplift models are correct, then as the peak ring collapsed
and deformed outward, it may have overrun and flowed over
surficial melt-bearing components before the fallback breccia
with its sedimentary clast components landed. The repetition
of "impact" melts within the granitic sequence implies there
was internal shearing within the granite as it displaced outward,
allowing it to cover breccias at least twice."
Preliminary chemical Data for drill cores
of the Chicxulub impact structure's peak ring
A. Wittmann et al. -- Lunar and Planetary Science XLVIII
(2017); 2075.pdf
"Upper Peak Ring
The 50 samples from the 130 m thick section of "suevite" and
(impact) melt rocks between 618.22‒744.07 mbsf show pronounced
vertical variation in their major and minor element concentrations.
Based on these chemical variations, the upper (peak ring)
section can be subdivded into an upper sorted "suevite" section
from 617.34‒684 mbsf that is grossly chemically homogenous.
A lower sorted "suevite"section from 687‒718 mbsf shows more
chemical variation compared to the subsection above, while
average concentrations are grossly similar.
The lowermost section between 720‒744 mbsf is dominated by
melt rocks and is chemically distinct with higher concentrations
of SiO2, TiO2, Al2O3, FeO compared to the "suevite" subsections
and notably, oxide totals that are typically 20 wt% higher
than those of the "suevite" subunits above.
Lower Peak Ring:
We analyzed 194 samples from the 586 m thick section of
granite, subvolcanic dikes, "suevite"
and melt rocks between 747.89‒1332.75 mbsf. The granite
lithologies in this section are remarkably similar in composition,
suggesting compositions of granites and syenites in the TAS
diagram and metasedimentary protoliths. Subtile differences
are present, though, for example Na2O concentrations from
748.89‒948.39 mbsf are lower than in the section between 948.39
and 1332.75 m. Very low incompatible trace element compositions
suggest a volcanic arc setting for the emplacement of these
granites, which is also supported by their magnesian character.
Compared to the granitic rocks, the most common intrusive
rock, a dark, aphanitic, subvolcanic lithology, has much lower
SiO2 and K2O contents, while MgO, FeO, CaO, MnO, TiO2 are
significantly enriched. A less common, brown, fine-grained
subvolcanic lithology that exhibits shatter cones displays
a chemical affinity to the dark subvolcanic dike lithology
in that it shows similar depletion and enrichment trends for
the major, minor and trace element concentrations compared
to their granite host rocks. In the
TAS diagram it plots in the field for phonotephrite, while
the dark subvolcanic dikes plot in the fields for foidite
and basanite.
Intercalcations of "suevite" and melt rocks show variable
compositions but tend to be relatively depleted in K2O and
enriched in TiO2, CaO, FeO, MgO, and MnO compared to the granitic
rocks."
Ages and Geochemistry of the Basement Granites
of the Chicxulub impact crater
L. Xiao et al. -- Lunar and Planetary Science XLVIII
(2017); 1311.pdf
"The granitic rocks are intruded
by three or more types of subvolcanic
dikes or dike swarms including felsite/phonotephrite, dacite/trachyte
and diabase/dolerite. Major rock-forming minerals of
the granitoid are alkali feldspar, plagioclase, quartz and
biotite. Accessory minerals observed in thin section include
zircon, apatite, sphene, and opaques. Shock metamorphism is
extensive throughout the core; planar deformation features
developed in quartz and plagioclase at mineral scale.
Five samples taken from depths of 829mbsf, 927mbsf, 979mbsf,
1076mbsf and 1200 mbsf were dated. Their 206Pb/238U ages
are 304+10Ma,321+7.7Ma, 313+14Ma, 325.9+7.5Ma and 340.8+9.9
Ma. A total of 147 analyses yielded major element oxide contents
for SiO2 of 64wt% to 79wt%, Al2O3 of 10 to 17wt%, Na2O of
3.5wt% to 7wt%, and K2O of 2wt% to 8wt%. In the TAS diagram,
most samples plot within the granite field, while some are
in the quartz monzonite and syenite field.
The zircon dating results suggest that
the granites were formed mostly around 300-340 Ma ago.
This large age variation of the five samples may indicate
that this was not a single granitic intrusion
during the
Carboniferous period."
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