The work I completed for a M.S. complemented part of a larger NSF-funded
project that P.I. Dr.
Jane Selverstone (UNM), Dr.
G. Lang Farmer (CU-Boulder), and John Aleinikoff (USGS-Denver) have
been working on for several years. The project involves understanding the
tectonic evolution of the Paleoproterozoic Colorado province by looking
at exposures in the northern Front Range of Colorado.
I focused on a research area west of Ft. Collins, Colorado, in Paleoproterozoic
supracrustal and igneous rocks. I concentrated on an east-west striking
package of deformed mafic metavolcanic rocks (now lower amphioblite grade)
that is surrounded by less-deformed metasediments to the north and south.
The mafic rock package hosts a sub-vertical shear zone and contains what
we interpret to be dismembered ophiolitic lithologies. The presence of
gabbros and clinopyroxenites with preserved yet highly deformed igneous
assemblages allows us to 'look through' both 1.7 and 1.4 Ga metamorphic
episodes and document igneous deformation that pre-dates the later metamorphic
episodes. Igneous fabrics include spectacularly deformed clinopyroxenes
that define horizontal lineations throughout the shear zone. Similar fabrics
and microstructures are known only from modern transform-fracture zones
and obducted transform fault ophiolites, and lead us to believe that the
Buckhorn Creek shear zone may have originated in such an environment. We
believe this mafic shear zone, and likely others in the northern Front
Range, are reactivated transform fracture zones that developed during ~2.1
Ga rifting of the Wyoming, Superior, and a third Archean province.
What I refer to as the Buckhorn Creek shear zone (field
shots) runs continuously for at least twenty kilometers from
Redstone Canyon (Horsetooth Reservoir quad) through the Buckhorn and Crystal
Mountain quads, and possibly into the Pingree Park quad where the cross-cutting
relationship with the Skin Gulch shear zone is unclear.
Kinematics: In thin section I have documented both igneous and metamorphic
lineations.
Igneous lineations are defined by elongate clinopyroxenes from deformed
gabbros, and also from relatively undeformed clinopyroxenes in clinopyroxenites
(see image below). In both rock types the pyroxenes define horizontal lineations
throughout the shear zone. Based on well-developed exsolution lamellae
developed along the length of the elongate pyroxenes, we believe the pyroxenes
first deformed during crystallization in supra-solidus conditions, and
then the lamellae developed during cooling.
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The metamorphic lineations consist of amphiboles in steep and shallow orientations, commonly within a single thin section. It is not easy to determine which lineation is related to 1.7 or 1.4 Ga deformation, and my work only touches on understanding this later history. We think the steep lineation is likely related to 1.4 Ga contraction.
XRF: I conducted whole-rock analyses for major and trace element determinations
on over 30 samples from the mafic body and surrounding 'lenses'. All of
the samples analyzed are clearly tholeiitic, and trace element discriminant
diagrams consistently indicate an oceanic origin, but not a unique one.
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It was only when we started to compare our analyses with basalts from
modern oceanic environments that we found similar geochemical trends with
mid-ocean ridge and fracture zone basalts, and were easily able to distinguish
our samples from trends developed in island-arc basalts. The modern data
set was graciously taken (for free) from the RIDGEPET
database , an impressive store-house of thousands of chemical whole-rock
analyses provided by ODP, DSDP, and many other sources. An incredible amount
of data can be down-loaded from the internet in a usable format in very
short time periods. These data helped us do comparissons like the one below
(e.g. TiO2 vs. MgO) using massive numbers of analyses.
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Stable Isotopes: I have only analyzed a few mineral pairs from the amphibolites (where garnet occurs) and more from the metasediments (typically garnet +/- aluminosilicate assemblages) for thermometric determinations on samples from north, south and within the shear zone. Most of the stable isotope work was done on quartz veins (see below) rather than on the mafic rocks.
Microprobe: We've analyzed clinopyroxenes and also the exsolution lamellae from the mafic minerals. The exsolution lamellae analyses are consistenly sub-calcic, but are not pure orthopyroxene.
My results rely heavily on geochemistry, field relations, and kinematics
to try and determine the protolith origin, emplacement mechanisms, and
accretionary history for this region of the Colorado province.
See publication list for references of this work.
