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This report summarizes the geology, concentrating on the Proterozoic rocks, of a 1,280-square-mile (3,300-square-kilometer) area comprising the northern Park Range of northernmost Colorado. Techniques are developed that are applicable to crystalline rocks of many kinds in other areas. The summary is supported by study of more than 2,300 thin sections collected during some 20 man-seasons of field work between 1965 and 1976, by 66 new rock and 10 new mineral chemical analyses, and by 36 other published analyses of Park Range rocks. The Precambrian rocks are about half sillimanite-grade, 1.8-billionyear (b.y.) -old metavolcanic and metasedimentary layered rocks, and half former magmatic rocks that intruded the layered rocks between 1.8 and 1.4 b.y. ago. The old layered rocks consist of 44 percent amphibolites, 37 percent felsic gneisses (both considered to be either volcanic rocks or sediments derived from a nearby volcanic source), 13 percent sediments (mainly metashales) from a chemical or distant clastic source, and 6 percent other or uncertain. In the last category are siliceous pod rocks containing 2-25 percent quartz-sillimanitemuscovite pods set in a matrix of quartz, oligoclase, microcline, biotite, and muscovite. Some of the pods are late metamorphic differentiates but others may be recrystallized relict primary structures of various types. Metamorphism of the old layered rocks proceeded in three stages: 1. Progressive regional dynamothermal metamorphism (1.8-1.7 b.y); 2. Static, purely thermal regional metamorphism (1.7-1.4(7) by.), during which many mineral species formed poikiloblastic crystals of great fragility; 3. Geographically localized dynamic metamorphism (1.4(?)-1.2 b.y). Thermal peak (550° -700°C, 1.7 b.y.) metamorphic mineral assemblages in metashales that have excess alumina are regionally arranged in increasing grade from one small area of a sillimanite-staurolitemuscovite assemblage, through successive broad areas of sillimanitemuscovite and sillimanite-muscovite-potassium feldspar assemblages, to three small maxima of a sillimanite-potassium feldspar assemblage. One tiny portion of the lowest grade area contains all three aluminum silicate polymorphs in equilibrium with staurolite and muscovite. Two areas of chloritoid schists outside of the lowest grade staurolitecontaining zone are believed to be compositionally controlled. Wherever aluminous metashales of sillimanite grade occur, clinopyroxene, garnet, and scapolite are found in intercalated mafic or calc-silicate rocks. Rocks of a rare third composition (high Mg, low Ca) contain anthophyllite and gedrite. Cordierite occurs in some representatives of both aluminous metashales and anthophyllite-gedrite rocks, giving the area some aspects of both Barrovianand Abukuma-type facies series (but no exact match for either). The regional pattern of isograds bears no simple relation to the areas or contacts of preorogenic or synorogemc gabbroic, quartz monzonitic, and quartz dioritic intrusive bodies. Tables summarize 190 theoretically possible contact relations between each of 20 recognized types of Precambrian intrusives. Most of these separate intrusive types are grouped in three main rock series: 1. 1.8-b.y. gabbro of Elkhorn Mountain in the northwest part of the area, originally intruded in separate but contiguous inhomogeneous batches and now consisting of: a. Peridotite rafts(?) in olivine gabbro containing coronas around olivine (6 percent); 6. Gabbros (60 percent; normatively containing olivine but modally free of olivine) to diorite (33 percent); c. Late, volumetrically minor basalt dikes. 2. 1.7-b.y. quartz monzonite of Seven Lakes in the northern half of the area; 1.7-b.y. quartz monzonite of Buffalo Pass in the southern half of the area (these first two occurring in about equal proportions); and undated, volumetrically minor quartz diorite of Gilpin Lake, locally gradational with quartz monzonite of Seven Lakes but possessing chemical affinities (possibly due to assimilation) with gabbro of Elkhorn Mountain. (Details of plagioclase distribution in different quartz diorite bodies suggest some unique sequential intrusive history.) 3. 1.4-b.y. quartz monzonite porphyry of Rocky Peak, quartz monzonite of Roxy Ann Lake, and other phases of the Mount Ethel pluton in the middle of the northern Park Range. All the intrusive rocks except this last series, which is believed to be younger than the thermal-peak assemblage described above, show visible mineral modification due to metamorphism. Volumetrically minor pegmatites, miscellaneous mafic intrusives, and small, isolated peridotite bodies resist organization in the above categories, but metamorphic remobilization and recrystallization have affected many of these rocks too. Corona minerals, developed by reaction between olivine and plagioclase in olivine gabbro and peridotite, generally occur in parallel layers or shells in the following order: 1. Olivine 2. Comb orthopyroxene (perpendicular to olivine contact) 3. Clinopyroxene 4. Light-colored clinoamphibole 5. Clinoamphibole-spinel symplectite (symplectite bodies perpendicular to plagioclase contact) 6. Plagioclase 1 PRECAMBRIAN PETROCHEMISTRY, NORTHERN PARK RANGE, COLORADO For example, electron-probe analysis of one small websterite body showed the following succession: 1. Hyalosiderite 2. Bronzite 3. Fassaite 4. Pargasite 5. Pargasite-pleonaste symplectite 6. Bytownite A detailed survey of 1:48,000 Park Range maps and of unmapped traverse observations and the assignment of relevant rock analyses to four major times of crustal increment have greatly improved our understanding of the changes in weighted Park Range chemistry in the 1.8to 1.4-b.y. time period. The average composition of Park Range rocks has never strayed far from a low-silica granodiorite (60-64 percent SiO2; normative plagioclase An41 _go), despite some inputs much different from this. When Park Range analyses are applied to a detailed statewide survey of exposed rocks of similar age (excepting the Uncompahgre Formation), a similar result is obtained. Average Colorado rocks have been slightly more silicic and slightly less mafic than average Park Range rocks at comparable times in their history, and alumina and alkali contents of these two rock types have been roughly comparable. Like the Park Range analyses, the statewide values cluster near a composition of low silica granodiorite (SiO2 62-66 percent; normative plagioclase An35_4g). If the Precambrian crust of either the Park Range or Colorado as a whole were melted completely, and stirred thoroughly, it would supply a rhyodacitic or quartz latitic magma Average Park Range Precambrian and average Colorado Precambrian compositions are compared with worldwide averages for Precambrian shield, continental crust, and lithosphere compiled from the literature. As might be expected, both the Park Range and Colorado averages are most comparable with the shield average Similarly, the average composition of Park Range peridotite (46 percent SiO2, 26 percent MgO) is close to the average of 11 separate estimates of upper mantle composition (45 percent SiO2 , 38 percent MgO) compiled from the literature. From the above data, seven chemical models are constructed to test the chemical constraints necessitated by derivation of various average crusts of known thickness from various average mantles. It is shown that, given the K2O values of the respective models and after correction for the Earth's sphericity, the minimum thicknesses of the model mantles required to derive the specified thicknesses of the model crusts after an average partial melt of about 3 percent range from 376 to 1,580 kilometers. These estimates assume that the mantle could be completely purged of K2O in the crust-forming process. Similar limiting conditions are imposed successively by Na2O, A12O3, and other compounds (assuming that earlier limiting conditions have been met in some way), and calculations are made and a graph is provided to facilitate similar calculations with any model. For all the models chosen, unrealistic mineralogy limits partial melts to a maximum near 25 percent, and the course of differentiation of the progressively depleted mantle residuum is toward normative olivine and away from normative plagioclase and pyroxene (especially away from normative orthopyroxene). Similar results would be expected for any other reasonable crust-mantle chemical models, thus casting doubt on models that require an increase in orthopyroxene in depleted mantle residua INTRODUCTION This report, based on field studies completed between 1965 and 1976, examines the implications of petrographic and chemical descriptions of Park Range Precambrian rocks not dealt with in several previous reports by the senior author. It builds on these preceding reports and should be used in conjunction with them to fully understand the petrochemical nature and problems of the area. Detailed geologic maps and cross sections of the area have been published (Snyder, 1980a, 1980b, 1980c). Adjoining areas of Precambrian rocks have been mapped by Steven (1954), Houston and others (1968), and Karlstrom (1977) to the northeast, and by Divis (1976), Houston and Ebbett (1977), Houston, Karlstrom, and Graff (1979), and Graff (1978) to the north. The Colorado geologic setting is shown on the summary maps of Ogden Tweto (1976a and b, 1978, 1979). The intrusive rocks in the central third of the area of this report, particularly the 1.4-b.y. Mount Ethel pluton, are discussed in detail in Snyder (1978) and are therefore touched only briefly in the present report. The general and economic geology of the northern two-thirds of the area is discussed in Snyder and others (in press). Northern Park Range radiometric geochronology by Carl E. Hedge and Charles W. Naeser is described in Segerstrom and Young (1972) and in Snyder (1978; 1980a, b, and c). Additional geochronology in nearby areas is summarized by B

Precambrian petrochemistry of the northern Park Range, Colorado, and its implications for studies of crustal derivation