Hornblende and biotite /sup 40/Ar//sup 39/Ar age spectra from rocks in south-central Connecticut help define a Permian-Triassic cooling curve for the area. Together with petrologic and structural information, a time-temperature-pressure-strain path is established. Similar data for the Narragansett basin in Rhode Island and Massachusetts allow correlation of the late Paleozoic histories of the two areas. Together, these data suggest that in the late Paleozoic, south-central New England was part of a fold-thrust belt, and the Narragansett basin was a retroarc foreland basin. NW-SE compression during the final assembly of Pangaea resulted in SE directed thrusting, causing the development of clastic wedges in adjacent Rhode Island and Massachusetts in the Late Carboniferous-Early Permian. A clockwise rotation of this deformation from NW to NNE led to northward underthrusting and concomitant uplift of both eastern Connecticut and Rhode Island in the Permian and Triassic.

It is proposed that many summit basins along the Aleutian Arc form from the clockwise rotation of blocks of the arc massif. Summit basins are arc-parallel grabens or half-grabens formed within the arc massif and are commonly located near or along the axis of late Cenozoic volcanism. Geomorphically, the Aleutian Arc appears to consist of contiguous rhombic blocks of varying size, tens to hundreds of kilometers in length. The boundaries between adjacent blocks are delineated by fault-controlled canyons that cut the southern slope of the arc transverse to its regional trend. Evidence that these blocks have rotated clockwise is provided by the triangular-shaped summit basins bordering the blocks to the north, oblique physiographic trends, offsets in the summit platform, and broad deflections in the southern slope of the arc. We present a model for block rotation that involves translation of blocks parallel to an arc. It is suggested that block rotation, which appears to have accelerated in late Cenozoic time, is linked to (1) a shift in the Euler pole for the Pacific plate, (2) the consequential start-up of late Cenozoic volcanism, (3) improved interplate coupling instigated by sediment flooding of the Aleutian Trench, and (4) westward subduction of northeast triking segments of the inactive Kula-Pacific Ridge.

Early Paleozoic plutonism in the central Virginia Piedmont consists of the igneous Lahore Complex and Ellisville Pluton. The Lahore Complex consists of a small altered mafic pluton intruded by the shoshonitic, alkalic monzonites of the Lahore Pluton (-450 Ma) that, in turn, is intruded by the calc-alkaline, granodioritic Ellisville Pluton (-440 Ma). These plutons were emplaced at about 760aC at a depth of 12 to 18 km within greenschist-facies rocks and are enclosed by contact-metamorphosed rocks.

Strongly contrasting pressure‐temperature‐time paths for the Avalon composite terrane and the structurally overlying Putnam‐Nashoba zone in eastern New England obtained from thermochronologic and thermobarometric data are best explained by a late Paleozoic underthrusting of cover rocks by the Avalon composite terrane. We present new Ar and U‐Pb thermochronologic data that show that in the southern Hope Valley zone, Permian (280 Ma) anatectic metamorphic conditions of 700°C and 6 kbar were quenched by relatively rapid cooling (12°C/m.y.) and exhumation (0.5 km/m.y.) for ∼40 m.y. In contrast, peak metamorphic conditions in the Putnam‐Nashoba zone predate Silurian intrusions, and slower cooling (3.5°C/m.y.) began at about 400 Ma. One‐dimensional thermal modeling suggests that these two belts were not in thermal equilibrium during the Permian metamorphism of the Avalon composite terrane. Because of the absence of high‐grade Alleghanian metamorphism in rocks overlying the Avalon terrane, we conclude that high‐grade Alleghanian metamorphism in the Avalon terrane occurred east of rocks now overlying it and that significant motion between Avalon and this cover occurred after peak Alleghanian metamorphism. Similarly contrasting metamorphic histories between Avalon inliers (Willimantic window, Massabesic complex gneiss, Pelham dome) and their cover rocks reveals the regional significance of this boundary. The core rocks all show Permian cooling, but the cover rocks show post‐Acadian cooling ages decreasing from east to west to the Pelham area, where hornblende ages in Avalon and cover differ by only 35 rather than 80 m.y. Model calculations show that thermal equilibrium between instantaneously thrusted blocks of rocks is generally obtained in tens of millions of years. Consequently, underthrusting of Avalon is constrained to be middle Mississippian or younger. Because the leading edge of the underthrusting block would have been heated the longest and would have most closely approached thermal equilibrium with its cover, core rocks of the Pelham dome must have been relatively close to this leading edge. Thus Carboniferous to Permian underplating from a generally eastward direction best explains these thermochronologic relationships.

The mineralogy of shocked mineral and lithic grains in the Cretaceous-Tertiary (K-T) boundary claystone worldwide is most consistent with a bolide impact on a continent. Both the concentrations and sizes of these shocked grains are greatest in the western interior of North America. These data suggest that the Manson impact structure in north-central Iowa is a viable candidate for the K-T boundary impact event. Argon-40-argon-39 age spectrum dating of shocked microcline from the crystalline central uplift of the Manson impact structure indicates that there was severe argon-40 loss at 65.7 +/- 1.0 million years ago, an age that is indistinguishable from that of the K-T boundary, within the limits of analytical precision.

40 Ar/ 39 Ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history (thermochronology) of the Panasqueira, Portugal, tin-tungsten deposit, a deposit spatially associated with a belt of Hercynian plutons. Muscovite samples with an age difference as small as 2.2 m.y. (0.7% of the age) are statistically distinct. Statistics are even better for comparison of multiple samples from separate events; that is, a difference of 0.9 m.y. (0.3%) can be resolved in this approximately 300-m.y.-old deposit. The major tin and tungsten ore-forming stages, which are the oxide-silicate stage, the main sulfide stage, and greisenization, occurred between 296.3 + or - 0.8 (1 Sigma ) and 291.6 + or - 0.8 m.y. (1 Sigma ). The first substage of the oxide-silicate stage was a short-lived thermal pulse at 296.3 + or - 0.6 m.y.; the fluids responsible may have emanated from the known granite cupola. The main sulfide stage was active at 294.5 + or - 0.9 m.y. as a slightly longer lived pulse with oldest evidence for this stage (295.8 + or - 0.6 m.y.) coming from areas farthest away from the known cupola and youngest evidence (293.5 + or - 0.8 m.y.) closest to the cupola. A second substage of the oxide-silicate stage occurred as a short-lived thermal pulse at 292.9 + or - 0.7 m.y., synchronous with greisenization of the cupola and alteration of the silica cap at 292.1 + or - 0.4 m.y. The duration of activity of the oxide-silicate stage, the main sulfide stage, greisenization, and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4.2 + or - 0.5 m.y. (1 Sigma ). Minor argon loss from all dated muscovites occurred during later reheating, probably during the longer lived pyrrhotite alteration stage. A single center, the known cupola, had a prolonged role and was the source for main sulfide stage, oxide-silicate stage II, greisenization, and alteration of the silica cap and possibly oxide-silicate stage I and the pyrrhotite alteration stage; however, a separate source for these latter two stages cannot be ruled out.This study is an example of a new and powerful application of 40 Ar/ 39 Ar age spectrum dating of muscovite. Because of the high precision demonstrated in this study, it is now possible to establish time constraints necessary for solving some of the long-standing problems in economic geology. Beyond this, the unique geologic situation of Panasqueira has allowed us to quantify the thermal characteristics of muscovite. Published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees C during rapid cooling or short reheating and a temperature of approximately 270 degrees C during slow cooling or extended reheating. Argon-loss patterns displayed by all dated muscovites resulted from reheating after original closure; the mechanism for this argon loss appears to have been argon transport by volume diffusion. Thus, 40 Ar/ 39 Ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age, duration, and number of episodes of mineralization.

This study was undertaken to obtain reconnaissance isotopic ages on tin, mercury, fluorine, and antimony mineralization in northern Mexico and to determine their temporal relation to the metallogenic evolution of the continental volcanic arc in this region. Deposits were dated using conventional K-Ar and 40 Ar/ 39 Ar analyses of whole rocks and phenocryst separates for altered and unaltered pre- and postore samples. Ore-related rhyolites in three tin deposits (Sombrerete, La Ochoa, and Cerro de los Remedios) formed at 30 to 31 m.y. ago and mineralization probably took place at the same time. The Guadalcazar stock, which hosts disseminated cassiterite, formed at about 28 m.y. ago. Mineralization in the Canoas mercury district formed at 26 to 27 m.y., although mineralization in the other two districts studied (Sain Alto and El Cuarenta) could be limited only to post-37 m.y. and post-40 m.y., respectively. Fluorite deposits hosted by limestone in the Zaragoza and Rio Verde districts formed at 30 to 31 m.y., and smaller, volcanic-hosted deposits in the Inde district could have formed as early as 38 m.y. The only radiometric age related to an antimony district obtained in this study was 36 m.y. for the granodiorite stock at Santa Maria de la Paz, north of the Wadley district. These ages demonstrate that the Sn-Hg-F-Sb mineralization was essentially contemporaneous with the latest phases of base and precious metal mineralization in northern Mexico and suggest that formation of deposits containing these metals is a special stage in the metallogenic evolution of volcanic arcs, which requires a relatively thick crust that is conducive to extensive magmatic differentiation and silicic volcanism.

Many Lower Paleozoic limestones and dolostones in the Valley and Ridge province of the central and southern Appalachians contain 10 to 25 weight percent authigenic potassium feldspar. This was considered to be a product of early diagenesis, however, 40Ar39Ar analyses of overgrowths on detrital K-feldspar in Cambrian carbonate rocks from Pennsylvania, Maryland, Virginia, and Tennessee yield Late Carboniferous-Early Permian ages (278–322 Ma). Simple mass balance calculations suggest that the feldspar could not have formed isochemically, but required the flux of multiple pore volumes of fluid through the rocks, reflecting regional fluid migration events during the Late-Paleozoic Alleghanian orogeny.

Mississippian Fossils from Southern Appalachian Metamorphic Rocks and Their Implications for Late Paleozoic Tectonic Evolution

The shallow-water limestones and dolostones of the Conococheague Limestone (Upper Cambrian) of western Maryland contain large amounts of authigenic potassium feldspar. The presence of halite daughter crystals in breached fluid inclusions, low whole-rock ratios of chlorine to bromine, and thermochemical data suggest that the potassium feldspar formed at low temperature by the reaction of connate brines with intercalated siliciclastic debris. Analyses of argon age spectra indicate that the authigenic feldspar probably formed during Late Pennsylvanian to Early Permian time. These results may indicate mobilization and migration of connate brines brought about by Alleghanian folding. The widespread occurrence of authigenic potassium feldspar in Cambrian and Ordovician carbonate rocks throughout the Appalachians suggests that this may have occurred throughout the entire basin.

Research Article| June 01, 1984 U-Th-Pb, Rb-Sr, and Ar-Ar mineral and whole-rock isotopic systematics in a metamorphosed granitic terrane, southeastern California ED DeWITT; ED DeWITT 1Geology Department, The Pennsylvania State University, University Park, Pennsylvania 16802 Search for other works by this author on: GSW Google Scholar RICHARD L. ARMSTRONG; RICHARD L. ARMSTRONG 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1W5 Search for other works by this author on: GSW Google Scholar JOHN F. SUTTER; JOHN F. SUTTER 3Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 Search for other works by this author on: GSW Google Scholar ROBERT E. ZARTMAN ROBERT E. ZARTMAN 4Branch of Isotope Geology, U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information ED DeWITT 1Geology Department, The Pennsylvania State University, University Park, Pennsylvania 16802 RICHARD L. ARMSTRONG 2Department of Geological Sciences, University of British Columbia, Vancouver, British Columbia, Canada V6T 1W5 JOHN F. SUTTER 3Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 ROBERT E. ZARTMAN 4Branch of Isotope Geology, U.S. Geological Survey, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1984) 95 (6): 723–739. https://doi.org/10.1130/0016-7606(1984)95<723:URAAMA>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ED DeWITT, RICHARD L. ARMSTRONG, JOHN F. SUTTER, ROBERT E. ZARTMAN; U-Th-Pb, Rb-Sr, and Ar-Ar mineral and whole-rock isotopic systematics in a metamorphosed granitic terrane, southeastern California. GSA Bulletin 1984;; 95 (6): 723–739. doi: https://doi.org/10.1130/0016-7606(1984)95<723:URAAMA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Mesozoic structural domes are developed in an older Proterozoic crystalline basement of granitic to granodioritic foliate metaplutonic rocks in the Halloran Hills, southeastern California. Isotopic analyses of whole rocks and mineral separates from these rocks by U-Th-Pb, Rb-Sr, and Ar-Ar techniques yield a complex pattern of discordance that is the result of a fairly simple geologic history. Individual mineral isotopic systems have variably equilibrated with each other in response to Mesozoic regional metamorphism and locally to later heating during Mesozoic batholith emplacement.Discordant U-Th-Pb zircon data indicate that the granitic core rocks are 1,710 Ma and that one dioritic phase may be slightly older. Rb-Sr whole-rock model dates scatter about 1,700 Ma Rb-Sr amphibole–whole-rock and U-Th-Pb amphibole dates are also Proterozoic. Potassium feldspars retain a 207Pb/206Pb signature of their Proterozoic age. Ar-Ar amphibole spectra from the flank of the main dome reveal disturbed dates of 1,450 Ma to 1,100 Ma, and the dates become younger toward the structurally deeper core of the dome.All remaining isotopic determinations yield Mesozoic or younger dates for mineral–whole-rock systems. Rb-Sr whole-rock–apatite–feldspar–biotite analyses show nonequilibration of strontium isotopes, with resultant mineral pair dates from 4 foliate plutonic rocks ranging from 200 to 50 Ma. No single metamorphic age is indicated by the Rb-Sr data. Rb-Sr whole-rock–biotite dates are consistently younger than any other determinations and may be reduced by weathering or gain of nonradiogenic strontium from ground water.U-Pb sphene and apatite analyses from rocks that yield 1,710-Ma zircon dates are nearly concordant at 140 Ma. An amphibole from the structurally deepest rocks of the main dome that yield 140- to 150-Ma U-Pb sphene dates has an Ar-Ar plateau date of 144 Ma. The U-Pb sphene and Ar-Ar amphibole analyses are believed to be the best age estimate for the end of the highest-temperature phase of regional metamorphism. Th-Pb sphene and apatite dates and Ar-Ar biotite dates cluster at 90 ± 5 Ma as a consequence of regional cooling during Late Cretaceous time following extensive Mesozoic plutonism in the region at 97 to 90 Ma.We interpret the discordant mineral date patterns to have resulted from metamorphism of ∼1,700-Ma plutonic rocks during the Jurassic (≥ 140–50 Ma) and subsequent uplift and cooling to ∼200 °C at about 90 Ma. On the basis of this study, the isotope dating systems ranked in decreasing order of resistance to resetting are: U-Th-Pb zircon (concordia intercept) ≥ Rb-Sr whole rock ∼Rb-Sr amphibole ∼U-Th-Pb amphibole ∼Pb-Pb whole rock > Ar-Ar amphibole ≥ Rb-Sr sphene ≥ U-Pb sphene and apatite > Rb-Sr plagioclase-potassium feldspar-apatite > Th-Pb sphene and apatite ∼Ar-Ar biotite ∼U-Pb feldspars > Rb-Sr biotite. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

30- to 32-m.y.-old rhyolite flows and domes (host rhyolite) capped by rhyolitic ignimbrites (cap ignimbrites). Relatively low crystallization temperatures (700 degrees + or - 50 degrees C) in the host rhyolites, but higher (740 degrees + or - 40 degrees C) in the cap ignimbrites. The host are high silica (>74% SiO 2 ) that are metaluminous or slightly peraluminous; they have high K 2 O/Na 2 O ratios; are enriched in Sn, U, Th, and Rb; and depleted in Ca, Mg, Ti, P, Co, Ni, Ba, Sr, and Eu relative to high K calc-alkalic rocks. Rare earth element patterns exhibit marked negative Eu anomalies and a slight enrichment of light rare earth elements relative to the heavy elements. These characteristics suggest that the rhyolites formed as extreme differentiates in high-level magma chambers closely related to caldera development. Initial Sr isotope ratios of the tin range from 0.7053 to 0.7078. Differentiation was clearly the major factor in concentrating tin in the Mexican tin rhyolites.--Modified journal abstract.

In South America at latitude 47°S, till was deposited sometime between 7 m.y. and 4.6 m.y. ago, at a time when the local climate was colder than today's. During this same interval glaciers extended to sea level in southeast Alaska, and widespread cooling of the ocean surface in middle latitudes, worldwide marine regression and change in the oxygen isotopic composition of ocean water occurred. From the last three events, major late Miocene expansion of the Antarctic Ice Sheet has been inferred, on the assumption that the history of North Atlantic ice rafting precludes the existence of Northern Hemisphere ice sheets until 3 m.y. ago. This is disputed, first because precipitation in Antarctica would probably have decreased at temperatures below today's, second because the Antarctic Ice Sheet cannot expand appreciably until buildup of Northern Hemisphere ice sheets has lowered sea level, third because virtually no late Miocene sediments are present at the Labrador Sea DSDP sites that are critical to the reconstruction of North Atlantic ice rafting history, and fourth because the scale of late Miocene glaciation in Alaska is at least permissive for simultaneous buildup of ice at similar latitudes further east. If global ice sheet volume at the end of the Miocene was greater than it is today, by an amount that would have lowered sea level by several tens of meters, the excess ice did not accumulate in Antarctica, but in the Northern Hemisphere, chiefly in North America away from the Atlantic coast.

Research Article| December 01, 1982 Structural geology and 40Ar-39Ar geochronology of the Goldstone–Lane Mountain area, Mojave Desert, California ELIZABETH L. MILLER; ELIZABETH L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar JOHN F. SUTTER JOHN F. SUTTER 2Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 Search for other works by this author on: GSW Google Scholar Author and Article Information ELIZABETH L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 JOHN F. SUTTER 2Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1982) 93 (12): 1191–1207. https://doi.org/10.1130/0016-7606(1982)93<1191:SGAAGO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation ELIZABETH L. MILLER, JOHN F. SUTTER; Structural geology and 40Ar-39Ar geochronology of the Goldstone–Lane Mountain area, Mojave Desert, California. GSA Bulletin 1982;; 93 (12): 1191–1207. doi: https://doi.org/10.1130/0016-7606(1982)93<1191:SGAAGO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Metasedimentary rocks near Goldstone, California, consist in ascending structural order of (1) a calc-silicate hornfels unit; (2) a quartzite, schist, and radiolarian chert unit; and (3) a calcareous and pelitic sequence of rocks that contains minor basaltic or andesitic composition dikes, sills, and flows(?). Unit 1 is of unknown, but probably pre-Mesozoic age; unit 2 is lower Paleozoic, in part Ordovician, and is correlative to lower Paleozoic eugeoclinal rocks in the El Paso Mountains north of the Garlock fault; unit 3 is probably correlative to upper Paleozoic (Permian) basinal sequences in the El Paso Mountains. These rocks were strongly deformed during a progressive deformational event that caused isoclinal to tight folding about northwest-southeast–trending fold axes and northeast-dipping axial planes. Tectonic transport during this deformation was in a southwesterly direction. Similarity of structural trends to those in the El Paso Mountains suggests that the deformation in these two areas is the same age and predates intrusion of Permo-Triassic plutons in the El Paso Mountains.Regional relationships suggest that deep-water metasedimentary rocks in the northwestern Mojave Desert and in the El Paso Mountains are allochthonous with respect to miogeoclinal and cratonal Paleozoic rocks elsewhere in the Mojave. The emplacement of the allochthonous rocks postdates deposition of Permian strata and predates Permo-Triassic to earliest Triassic deformation and plutonism. This deformation involved both eugeoclinal and miogeoclinal sequences and took place within an Andean arc setting. The subsequent Mesozoic history of these two parts of the Mojave Desert has been the same.40Ar-39Ar data on hornblende, biotite, and muscovite in the Goldstone and Lane Mountain quadrangles indicate that a plutonic complex consisting of gabbro, quartz diorite, and tonalite was intruded during the Late Jurassic, probably slightly earlier than 148 m.y. ago. Voluminous granitic rocks were intruded during the Late Cretaceous, about 85–90 m.y. ago. These younger plutonic rocks caused variable resetting of all K-Ar systems in older rocks. Cooling of Late Cretaceous plutons to about 280 ± 40 °C occurred by about 78.4 m.y. Regional uplift of these rocks to a crustal level where no further diffusive loss of argon took place occurred at about 45 m.y. ago. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Research Article| April 01, 1981 40Ar/39Ar age spectra for biotite and hornblende from plutonic rocks in the Victorville region, California ELIZABETH L. MILLER; ELIZABETH L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar JOHN F. SUTTER JOHN F. SUTTER 2Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 Search for other works by this author on: GSW Google Scholar Author and Article Information ELIZABETH L. MILLER 1Department of Geology, Stanford University, Stanford, California 94305 JOHN F. SUTTER 2Department of Geology and Mineralogy, The Ohio State University, Columbus, Ohio 43210 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1981) 92 (4): 164–169. https://doi.org/10.1130/0016-7606(1981)92<164:AASFBA>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ELIZABETH L. MILLER, JOHN F. SUTTER; 40Ar/39Ar age spectra for biotite and hornblende from plutonic rocks in the Victorville region, California. GSA Bulletin 1981;; 92 (4): 164–169. doi: https://doi.org/10.1130/0016-7606(1981)92<164:AASFBA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 40Ar/39Ar data from plutonic rocks in the Mojave Desert near Victorville, California, provide preliminary insight into the complex thermal history of this region and suggest that some plutons here may be significantly older than any plutons in the Sierra Nevada to the north. Near Victorville, multiple deformation and metamorphism of platform-miogeoclinal strata is inferred to postdate deposition of Permian rocks and predate intrusion of monzonite plutons. Monzonite that intrudes multiply deformed Paleozoic marble and that is unconformably overlain by the Mesozoic (Triassic or Jurassic) Fairview Valley Formation has hornblende with an age spectrum typical of severe 40Ar loss. We interpret the age of the highest temperature increment (fusion), 233 ± 14 m.y., as a minimum age for this pluton. The total gas age for the hornblende is only 169.9 m.y. Biotite from the same rock exhibits a plateau age of 76.59 ± 1.24 m.y. The Fairview Valley Formation is overlain by volcanic rocks of the Sidewinder sequence. These rocks were folded and intruded by quartz monzonite that postdates all penetrative deformation in the region. Biotite-hornblende tonalite at Quartzite Mountain is part of one of these younger quartz monzonite plutons, and its biotite yields a plateau age of 73.16 ± 1.11 m.y. This age is the same at the 95% confidence level as the age of biotite from the older monzonite; thus, this younger thermal event was at least the partial cause for the argon loss pattern displayed by the monzonite hornblende. Hornblende gabbro that intrudes Paleozoic marble at Sidewinder Mountain and is in turn contact metamorphosed by later plutonic rocks has hornblende with a disturbed 40Ar/39Ar age spectrum. The 1025, 1050, and 1075 °C steps contain 44.64% of the total argon and yield a common age of 119.2 ± 3.4 m.y., which we interpret as the best estimate for the age of the hornblende. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.