Rocky Mountain Geology 34.2

Paleo- and Mesoproterozoic granite plutonism of Colorado and Wyoming

J.L. ANDERSON and R. L. CULLERS

Keywords: Proterozoic, granite, anorogenic, oxygen, isotopes, magnetite, ilmenite, peraluminous, metaluminous, rapakivi

Proterozoic plutonism in Colorado and Wyoming has initiated ~1.8 Ga with scattered tholeiitic mafic complexes coeval with widespread synorogenic bimodal volcanism. Limited Nd and Sr isotopic data for the metavolcanic rocks show derivation from depleted mantle. Major Paleoproterozoic granite pulonism followed at 1.67-1.77 Ga. Most of the earliest plutons are distinctly calc-alkaline; they range largely from quartz diorite to granodiorite to trondhjemite in composition, and have trace-element signatures similar to plutons within magmatic arcs related to subduction zones. Later Paleoproterozoic plutons at 1.71 Ga include increased volumes of felsic rock types and, independent of silica, are shifted to more peraluminous and iron-rich compositions. The earliest appearance of anorthosite occurs with the 1.76-Ga Horse Creek anorthosite complex of Wyoming, and the earliest occurrence of A-type granite includes the late-kinematic, 1.66-Ga Garell Peak batholith of southern Colorado. Elemental and isotopic compositions of the younger Early Paleoproterozoic granitic plutons are consistent with a systematically increasing crustal component as a function of age in waning orogenic stages of crust formation in the region. After a 200 m.y. hiatus, renewed granitic plutonism occurred at 1.36-1.44 Ga. Plutonism was associated with emplacement of over a dozen Mesoproterozoic A-type granite batholiths and many smaller intrusions as part of a global "anorogenic" mid-Proterozoic event that commonly includes associated instrusions of anorthosite and charnockite. Across the former Laurentia supercontinent, three geographic and petrologic subprovinces merge in Colorado and Wyoming. An ailmenite-series granitic province, which includes the Sherman Granite and associated Laramie anorthosite complex of Wyoming, extends northeastward through Wisconsin to Labrador and the classic rapakivi granite-anorthosite instrusions of the Baltic region. A magnetite-series granite subprovince ranges across the southern mid-continent to California and includes the San Isabel and Eolus batholiths of southern Colorado. The third subprovince is peraluminous, comprised of two-mica granite, and geographically extends from central Colorado to Arizona. Granites of this suite are the most common in Colorado and include the Silver Plume batholith. Granites defining the three Mesoproterozoic provinces have distinctly different elemental and oxygen isotopic compositions, which presumably reflect fundamental shifts in composition of the lower continental Laurential crust. The mid-proterozoic instrusions of Colorado and Wyoming coincided in time with emplacement of regional, north-trending mafic dike swarms, implying widespread extension during this period. After another magmatic hiatus, one of ~300 m.y., instrusion of the A-type, 1.08-Ga Pikes Peak batholith formed the last Proterozoic magmatic episode of the Colorado-Wyoming Front Range.

Petrogenesis and tectonic context of the Harney Peak Granite, Black Hills, South Dakota

PETER I. NABELEK, MONA SIRBESCU, and MIAN LIU

Keywords: Black Hills, Harney Peak Granite, Trans-Hudson orogeny, leucogranite, crustal collision, P-T-t paths, shear-heating, radiogenic isotopes, stable isotopes, metapelites.

Instrusion and crystallization of the Harney Peak Granite and associated plutons and pegmatites were the culminating events of the Trans-Hudson orogeny as expressed in the Black Hills. The granite were emplaced as thousands of sills and dikes at 1715 Ma, following as approximately 45 m.y. period of regional metamorphism and deformation of now exposed Proterozoic metasedimentary rocks. Isotopic ratios of neodymium and lead indicate that parts of the granite were derived from sourse rocks with Archean model T-DM extraction-ages, whereas other parts were derived from sources with only 100-300 m.y. crustal residence times. However, oxygen isotope ratios and trace element concentrations suggest that both sources were metapelites or metagraywackes analogous to the country rocks. Boron and Ti02 concentrations suggest that the granites were generated either by muscovite or muscovite + biotite dehydration-melting reactions. Published thermobarometric and argon cooling-ages show that the country rocks cooled and decompressed from conditions at which garnet and staurolite grew at ~7 kbar to less that 500 degrees Celcius and pressures of 3.5-4 kbar at the time of granite emplacement. We conducted a numerical simulation of metamorphism and generation of the Harney Peak Granite that is constrained by available data. The model involves shear-heating along a thrust to produce temperatures sufficiently high for melting of thrusted sedimentary rocks at relatively shallow levels ina thickened crust that is undergoing unroofing. The model successfully reproduces the metamorphic and magmatic events that occurred in the Black Hills segment of the Trans-Hudson orogen.

A geodynamic model for Paleoproterozoic post-tectonic magma genesis in the southern Trans-Hudson (Black Hills, South Dakota) and Penokean (southern Lake Superior) orogens

DANIEL K. HOLM

Keywords: Post-tectonic magmatism, Paleoproterozoic, mantle delamination, Penokean, Trans-Hudson

Previous attempts to explain Paleoproterozoic post-tectonic magmatism in the Penokean and southern Trnas-Hudson orogens inadequately account for the entire tectonic setting and geologic history of these areas. Key characteristics of post-tectonic plutonism in both orogens include the similar time-lag of emplacement after orogenesis (50-60 m.y.), the close association in both time and space between post-orogenic uplift and intrusion, the depth of emplacement, and long-lived tectonic quiescence (crustal stabilization) that followed magma emplacement. Post-tectonic magmatism appears closely associated with post-orogenic uplift (collapse) and crustal stabilization. I propose that convective removal of the mantle lithospheric root may best explain post-tectonic magma genesis. Such a model suggests that there is a simple and related evoluntionary history that began with construction of the Penokean and southern Trans-Hudson orogens (crustal thickening, metamorphism, plutonism, etc.) and ended with their destruction (crustal thinning, widespread uplift/cooling, plutonism, etc.), thus stabilizing the crust. Post-tectonic plutons may be an indelible mark of orogenic demise, not the culmination of terminal collision. Delamination-magmatism probably played a key role in the differentiation and stabilization of crust after orogenesis and may have promoted its transformation into craton.

Virginia Dale instrusion, Colorado and Wyoming Magma-mixing and hybridization in a Proterozoic composite intrusion

RODERICK W. VASEK and ALLAN KOLKER

Keywords: Magma mixing, hybridization, diorite, Cap Rock monzogranite, Proterozoic Sherman batholith, Virginia Dale instrusion, Colorado, Wyoming

The Virginia Dale instrusion is a granitic and monzogranitic composite intrursive body at the sourthern extent of the Sherman batholith in southeastern Wyoming and adjacent areas of Colorado. Within the study area, at the margin of the intrusion, mafic rock (diorite) is commingled with felsic rock (monzogranite) and intermediate rocks. Resulting features include pillows (some with zones of felsic or mafic enrichment at their margins), enclaves, mantled quartz grains, and alkali feldspar megacrysts. Intermediate rocks interpreted as hybrids include: (1) granodioritic hybrid; (2) main-phrase hybrid; (3) granitoid hybrid; and (4) mafic hybrid. Each of the hybrid shows ditinct textual and mixing relations. Mixing and hybridization models were evaluated using major-element, trace-element, and isotopic data. Major-element compositions of the hybrid rocks generally plot along simple mixing lines between mafic and felsic rocks. Incompatible element concentrations of the intermediate rocks are similar to, or greater than, concentrations in the felsic end-member. Trace-element data suggest that the variety of intermediate rocks resulted from chemical diffusion in addition to bulk mixing of heterogeneous end-members. Mafic rocks show a possible differentation trend, in which the most primitive members are cumulates. The more evolved members show progressive LREE and incompatible element enrichment, growth of a negative europium anomaly (to Eu/Eu*=0.61), and a correlated decrease in Mg-number (to 0.35). Virginia Dale isotopic data are similar to values for the adjacent Sherman batholith, suggesting that both of these bodies formed by partial melting (at 1.43 Ga) of ca. 1.8-Ga crust in the Colorado province. Initial 87Sr/86Sr values are more heterogeneous, perhaps due to chemical diffusion.

1.4-Ga peraluminous granites in central New Mexico: Petrology and geochemistry of the Priest pluton

AMY G. THOMPSON and CALVIN G. BARNES

Keywords: Proterozoic, granite, petrology, geochemistry, peraluminous, crustal melting, New Mexico

The Priest pluton is part of the suite of 1.4-Ga granitic plutons in North America. The pluton is a corundum normative, peraluminous, K-feldspar megacrystic granodiorite to quartz monzonite. It is characterized by: (1) high Mg/Mg + Fe; (2) high abundances of Al2 O3, CaO, and large ion lithophile elements (e.g., Sr, Ba); and (3) low abundances of high field strength elements (e.g., Zr, Y). Major and trace element modeling suggests that variations in composition were produced by accumulation of K-feldspar and plagioclase, localized accumulation of plagioclase, biotite, and accessory minerals, and late-stage crystal fractionation of the residual melt. Plutons in the suite of 1.4-Ga, so called "anorogenic" granitic plutons have been classified as A-type granites. Major and trace element abundances suggest that the Priest pluton is not A-type, but have more in common with S-type granites. In addition, typical tectonic classification techniques suggest that the pluton is eithera volcanic-arc/syn-collisional or late orogenic granite. The pluton's intrusion was accompnied by highly partioned, compressional deformation that is not typically associated with anorogenic, A-type plutons. The Priest pluton aslo is distinct from the nearby Sandia pluton, although the two plutons are nearly the same age. The Priest pluton has higher Mg number, alumina saturation index, and large ion lithophile element abundances as well as lower abundances of high field strength elements. The Priest pluton contains only biotite as its mafic phase, whereas the Sandia pluton contains biotite and the suite of 1.4 Ga granitic plutons suggest that the lower crust is a heterogeneous source region. Therefore, geochemical signatures that commonly are associated with particular tectonic settings may instead reflect heterogeneities in the source.

Isotopic and elemental chemistry of subsurface Precambrian igneous rocks, west Texas and eastern New Mexico

MELANIC A. BARNES, C. RENEE ROHS, ELIZABETH Y. ANTHONY, W. RANDY VAN SCHMUS, and RODGER, E. DENISON

Keywords: Proterozoic, granite-rhyolite province, A-type granites, Nd model ages, gabbro, crustal growth, Laurentia, Texas, New Mexico

We present major element, trace element, and Nd isotopic analyses from cuttings and core samples for three subsurface terranes in West Texas and eastern New Mexico. The most northerly is the Panhandle volcanic terrance, which represents a large part of the Mesoproterozoic southern granite-rhyolite province. This terrane is comprised of undeformed rhyolite, ignimbritic tuff, granite, and diabase. The Panhandle terrane is split by the Debeca terrane, which consists of intercalated metasedimentary and metavolcanic rocks intruded by olivine gabbro, ferrogabbro, and diabase. Mildly to strongly deformed intermediate and felsic intrusive rocks of unknown affinity make up the third terrane, called here the crystalline terrane; it is located south and southeast of the Panhandle and Debeca terranes. Intermediate-to-felsic rocks of the terranes can be subdivided on the basis of their geochemistry into those with: (1) K2O/Na2O>1 and A-type trace element characteristics; and (2) K2O/Na2O<1 and I-type trace element characteristics. All but a few samples from the Panhandle terrane, both north and south of the Debaca terrance, are A-type. Depleted mantle model ages for granities and rhyolites from the northern Panhandle terrane range from 1.50-1.69 Ga. Two samples from the southern Panhandle terrance have model ages of 1.74 Ga, and a third sample's model age is 1.57 Ga. These model ages are older than the four reported crystallization ages of 1.37-1.40 Ga, indicating that: (1) A-type rocks of the Panhandle terrane contain a significant crustal component; and (2) Panhandle terrane is underlain by >1.7-Ga crust. The southern edge of Laurentia, therefore, is farther south than previously inferred. A diabase from the Panhandle terrane has a TDM of 1.44 Ga. If this model age is close to the crystallization age, then diabase in the Panhandle terrane is approximately coeval with the granite and rhyolite. The model age for the gabbro from the Debeca terrance is distinctly younger at 1.26 Ga, and is the same as crystallization ages of felsic tuffs associated with shelf carbonates in the Franklin Mountains and Van Horn area. In the crystalline terrane, both A- and I-type granites are present. Model ages for the I-type granites are 1.40-1.47 Ga. These are distinctly younger than the model age fro the Panhandle terrane, and an A-type granite has a TDM of 1.35 Ga. These data indicate that granites in the crystalline terrane are not part of the granite-rhyolite province; rather, they constitute a separate group.

Diverse Mesoproterozoic basaltic magmatism in west Texas

CALVIN G. BARNES, WILLIAM M. SHANNON, and HULUSI KARGI

Keywords: Mesoproterozoic, west Texas, ferrobasalt, theleiite, Grenville, layered intrustion.

Two distince basaltic suites characterize ~1165 to 1120-Ma magmatism in west Texas. The ~1163 Ma pecos mafic intrusive complex consists of layered mafic and ultramif rocks. Megacyclic units record repeated influx and mixing of tholeiitic magmas to yield a predominantly noritic body. Basaltic magmatism also accompanied emplacement of the 1120 +/- 35-Ma Red Bluff granitic suite in the Franklin Mountains. In terms of their major elements, these basaltic rocks are transitional between tholeiitic and alkaline, but their low concentrations of Nb suggest a subduction-related origin. Except for their low Nb contents, the Franklin Mountains basaltic rocks are similar to coeval basaltic intrusive and volcanic rocks in the southwestern U.S.A. Of the numerous proposed tectonic settings, an extensional environment is preferred. Possible extensional associations include splays of the Midcontinent Rift, rifting caused by collision of a continental fragment during the Grenville orogeny, and extension related to collapse of over-thickened crust during the Grenville event. The low Nb contents of basalts associated with Franklin Mountains basaltic rocks are best explained as an artifact of older subduction, which modified the mantle source of the basalts hundreds of millions of years prior to their emplacement.

Mesoproterozoic tectonic evolution of the western Llano uplift, central Texas: The story in an outcrop

ROBERT C. ROBACK, BRIAN B. HUNT, and MARK A. HELPER

Keywords: Llano uplift, Precambrian Grenville, Mesoproterozoic, U-Pb geochronology.

A small, but perfectly exposed outcrop of Mesoproterozoic rocks in the western Llano uplift displays lithologic and structural relations from which a sequence of geologic events has been determined. Detailed geologic mapping and petrologic studies combined with new U-Pb ages of key units are used to constrain the tectonic evolution of this little-studied region and provide a basis for comparison with the previously studied southeastern uplift. The outcrop consists of layered mafic and felsic gneiss cross-cut by three generation of granitic dikes and sills. The sequence of events as deduced from cross-cutting relations and U-Pb geochronology is as follows: (1) generation of compositional layering (So) and parallel biotite foliation (S1) in the gneisses at 1256 Ma; (2) emplacement of the first set of granitic sills and dikes at 1253 Ma; (3) transposition, isoclinal folding, amphibolite facies metamorphism, and fabric formation (S2) between 1243 Ma and 1126 Ma; (4) emplacement of pegmatic granitic dikes at 1126 Ma; (5) open folding and boudinage between 1126 Ma and 1076 Ma; and (6) emplacement of aplitic granitic dikes at 1076 Ma. Four U-Pb ages of titanite, two each from the gneiss and pegmatitic graniti, are concordant at ~1114 Ma indicating that the region had been sufficiently hot to reset titanite ages in the gneiss and that the region had cooled below the titanite blocking temperature by this time. The tectonic evolution of this outcrop is remarkably similar to that of other areas of the Llano uplift, suggesting that the entire uplift experienced a similar evolution.

A review of the Pikes Peak batholith, Front Range, central Colorado: A "type example" of A-type granitic magmatism

DIANE R. SMITH, JEFF NOBLETT, REINHARD A. WOBUS, DAN UNRUH, and KEVIN CHAMBERLAIN

Keywords: Colorado, geochemistry, petrology, granite, Proterozoic.

The ~1.08-Ga Pikes Peak composite batholith of central Colorado is a type example of an A-type granitic system. From the 1970s through the 1990s, details of the field relations, mineralogy, major and trace element compositions, and isotopic geochemistry of Pikes Peak rocks were documented, and they reveal the existence of two chemical groups, a potassic and a sodic series. The potassic series (~64-78 wt % SiO2) includes the Pikes Peak Granite, which is mostly coarse-grained biotite+/- hornblende syenogranite and minor monzogranite that dominates the batholith. The potassic series also includes fine-to-medium-grained biotite granite found in numerous, small late-stage plutons throughout the batholith. The sodic series is found in seven plutons comprised of a wide range of rock types (~44-78 wt % SiO2), including gabbro, diabase, syenite/quartz syenite, and fayalite and sodic amphibole granite. Differences in petrologic and geochemical characteristics between the sodic and potassic series indicate different peterogenetic histories. Major and trace element and strontium and oxygen isotopic data were used by some workers to hypothesize that mantle-derived alkali basalt underwent crystal fractionation and reaction with lower crustal rocks to generate syenitic magmas of the sodic series, which subsequently underwent further fractionation to produce sodic granites. Recent studies involving estimates of oxygen fugacities, along with additional trace element and neodymium isotopic data, also support a basalt fractionation model for the sodic series, but suggest only minor crustal involvement. Gabbros and diabase dikes associated with the sodic series appear to have been derived from mantle sources that previously had been affected by a subduction event, based on neodymium isotopic and trace element data. Some workers propose that the potassic series also formed by fracionation of syenitic and/or basaltic magmas coupled with reaction with intermediate rather than lower crust. Other workers propose a model in which genesis of the potassic series was dominated by partial meling involving tonalitic sources, with fractionation and perhaps magma mixing playing subordinate roles in generating compositional deversity among the potassic granitoids. The Pikes Peak batholith thus formed by emplacement of at least two petrogenetically different granite types, which were emplaced close together in space and time and which exhibit geochemical characteristics typical of A-type granites.

Petrogenesis of the Sugarloaf syenite, Pikes Peak batholith, Colorado

RACHEL BEANE and REINHARD A. WOBUS

Keywords: Sugarloaf Peak, Pikes Peak, syenite, granite, Colorado, pluton, fractional crystalizaion, Precambrian, geochemistry, REE.

Sugarloaf Peak is one of seven sodic plutons that lie within or adjacent to the ca. 1.08-Ga Pikes Peak batholith in central Colorado. This report represents the first study of the Sugarloaf pluton. Major element and modal analyses from the other six plutons (Lake George, Tarryall, Rampart Range, West Creek, Mt. Rosa, and Spring Creek), together with data presented here, indicate that sodic and potassic rocks from all of them were produced by fractional crystallization of mantle-derived basaltic magmas. The Sugarloaf pluton is composed of fine-grained, medium-grained, coarse-grained, and pegmatitic syenite. The syenites lack quartz and are dominated by perthitic feldspar and ferrorichterite amphibole. The fine-grained syenite intrudes the medium- and -coarse-grained syenites. The Sugarloaf pluton is surrounded by coarse-grained Pikes Peak Granite, which is the predominant rock type in the batholith. The linear arrangement of six of the seven sodic plutons parallel to major Precambrian fault trends suggests that the emplacement of Sugarloaf plutons may be rift-related. The Sugarloaf syenites have high total alkalis, high FeO (total), low CaO, and low MgO concentrations. They are also enriched in rare earth elements (REE) and high field strength elements (HFSE). Pronounced trace element variation among the Sugarloaf syenites can be explained partially by models of fractional crystallization. A plot of Ba versus Sr shows that compositions of the syenites closely follow modeled fractionation vectors for potassium feldspar. The fractional crystallization trends show that fine-grained syenite is the most chemically evloved, consistent with field relations that show the fine-grained syenite intruded the medium- and coarse-grained syenites. Accessory mineral fractionation, release of volatiles, or removal of pegmatitic fluids also may have influenced geochemical variations among the Sugarloaf syenites.