Geologic and isotopic investigations of the early Cretaceous Sierra Nevada Batholith, Tulare Co., CA, and the Ivrea Zone, NW Italian Alps: examples of interaction between mantle-derived magma and continental crust
15
Citation
0
Reference
20
Related Paper
Citation Trend
Abstract:
Two igneous suites containing layered ultramafic-mafic cumulates were investigated with the intent to characterize the parental magma and to identify processes significant to the petrogenesis of these rocks. In both study areas, the early Cretaceous Sierra Nevada batholith and the Ivrea Zone, isotopic systematics of the cumulates were found to preserve the characteristics of the mantle-derived parental magma and to record the effects of fractional crystallization and assimilation. Modeling the relative importance of these processes and characterization of the material derived from the mantle are necessary to understanding the growth of the continental crust. Geologic mapping of 110 mi2 of the 125 to 110 Ma Stokes Mountain region reveals the presence of layered cumulate megaxenoliths and two coeval ring dike complexes. Petrographic analysis and geochemical modeling of 125 dominantly mafic and intermediate samples demonstrate the comagmatic nature of this suite. Combined oxygen, strontium and neodymium analysis of 22 samples indicates, however, that each ring complex was fed by an isotopically distinct parental magma (eNd(115) = +6.1, Sri = 0.70338, δ18O = 6.6‰ ; (eNd(115) = +5.7, Sri = 0.70372, δ18O = 6.7‰) both of which were derived from a variably contaminated, depleted mantle source. Minor assimilation of continentally-derived metasediments and mafic-ultramafic material of the Kings-Kaweah ophiolite further affected the isotopic evolution of the two subsuites. Hydrothermal alteration in the subvolcanic environment is recorded only by rare stoped xenoliths of 120 Ma hypabyssal intrusives. Late Hercynian (≈300 - 270 Ma) magmatism produced the 10 km thick Mafic Complex lying at the base of the Ivrea-Strona-Ceneri crustal cross section. δ18O analysis of 237 whole rock samples and 26 mineral separates reveals that presumably early intrusions into the cool crust preserve the depleted mantle signature of the modeled parental magma (eNd(115) = +7, Sri = 0.703, δ18O = 6.5‰) while later intrusions assimilated significant amounts of the 10 - 12‰ metapelite. Subsequent intrusion of voluminous basaltic magma fonned a large, convecting magma chamber in which assimilation was concentrated within boundary layers. Such lower crustal production of high-18O (δ18O = 8 - 10‰) mafic magmas is suggested as contributing to the petrogenesis of upper crustal Permian granites.Keywords:
Batholith
Underplating
Fractional crystallization (geology)
Ultramafic rock
Research Article| November 01, 1973 Variations in Sr, Rb, K, Na, and Initial Sr87/Sr86 in Mesozoic Granitic Rocks and Intruded Wall Rocks in Central California RONALD W. KISTLER; RONALD W. KISTLER 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar ZELL E. PETERMAN ZELL E. PETERMAN 2U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information RONALD W. KISTLER 1U.S. Geological Survey, Menlo Park, California 94025 ZELL E. PETERMAN 2U.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 (1973) 84 (11): 3489–3512. https://doi.org/10.1130/0016-7606(1973)84<3489:VISRKN>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 RONALD W. KISTLER, ZELL E. PETERMAN; Variations in Sr, Rb, K, Na, and Initial Sr87/Sr86 in Mesozoic Granitic Rocks and Intruded Wall Rocks in Central California. GSA Bulletin 1973;; 84 (11): 3489–3512. doi: https://doi.org/10.1130/0016-7606(1973)84<3489:VISRKN>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 Initial Sr87/Sr86 of granitic rocks which are exposed north of the Garlock fault in California, and which represent the entire 130-m.y. time span of emplacement during the Mesozoic, ranges mainly from 0.7031 to 0.7082, with one value of 0.7094. A systematic areal variation, independent of age, exists for initial Sr87/Sr86 in these granitic rocks and is the same as the areal variation in initial Sr87/Sr86 of superjacent upper Cenozoic basalts and andesites.Two values of initial Sr87/Sr86, 0.7040 and 0.7060, mark natural separations of granitic rock data on K-Rb, K-Sr, and Rb/Sr-Rb variation diagrams, and also, when contoured, seem to represent geographic markers of paleo-geographic, geochemical, and physiographic significance. Upper Precambrian sedimentary and metamorphic rocks in California crop out only in the region where initial Sr87/Sr86 of granitic rocks is greater than 0.7060. A line of initial Sr87/Sr86 = 0.7060 is approximately coincident with the boundary between Paleozoic eugeosynclinal and miogeosynclinal rocks. Granitic rocks intruded into Paleozoic miogeosynclinal rocks have initial Sr87/Sr86 greater than 0.7060, whereas those intruded into eugeosynclinal Paleozoic rocks have initial Sr87/Sr86 less than 0.7060. The line of initial Sr87/Sr86 = 0.7040 is the eastern limit of principal exposures of ultramafic rocks, the western limit of Cretaceous granitic rocks, and is coincident with an abrupt change in "topographic expression" on the Bouguer gravity map of California. Correlation of the isotopic variations with these major crustal features suggests that there has been a sharp lateral contrast in crust-mantle chemistry across the region of study that has been fixed in position from the Precambrian to the present time.The chemical and isotopic variations observed are best explained if the parent magmas of the majority of granitic rocks investigated were derived in a region that was laterally variable in composition and in a zone of melting that intersected both upper mantle and lower crust. However, some igneous rocks, such as Jurassic volcanic rocks in wall rocks and roof pendants and some granitic rocks with high strontium concentrations and low Rb-Sr ratios, suggest that deeper sources are also involved in the total spectrum of igneous rocks in the region. 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.
Geological survey
Cite
Citations (400)
Hornblende
Diorite
Batholith
Sill
Phenocryst
Silicic
Cite
Citations (223)
The southernmost Sierra Nevada offers a view into the deep levels of the Mesozoic batholithic belt which constitutes much of the range to the north, and represents one of the major tectonic features of western North America. The main crystalline rocks of the study area are (1) the intrusive suite of Bear Valley, a middle Cretaceous tonalite batholith complex with coeval gabbroic intrusives, and (2) the gneiss complex of the Tehachapi Mountains, which consists of Early Cretaceous orthogneiss and subordinate paragneiss, with local domains having granulite facies metamorphic assemblages. The orthogneisses are dominantly tonalitic in composition, with significant layers of granodioritic to granitic and lesser dioritic to gabbroic gneiss. Quartz‐rich and psammitic metasedimentary rocks with subordinate marble constitute the main framework assemblage into which the plutonic rocks were emplaced. Field relations demonstrate assimilation of metasedimentary material into the orthogneiss and tonalite batholith magmas, and magma mixing between mafic, tonalitic, and granitic materials. Significant domains of both homogenization and inhomogenization are recognized isotopically within the mixed rocks. U/Pb zircon studies have resolved two major igneous suites and a third suite of postdeformational intrusives, all lying between 90 and 120 Ma. The first suite (gneiss complex of the Tehachapi Mountains) was emplaced at ∼115 Ma, and exhibits penetrative high‐temperature deformation developed at or near solidus conditions. A number of discordance patterns, along with the physical properties of the zircon, suggest minor inheritance of Proterozoic zircon and limited open system behavior in response to a major 100 Ma plutonic event. The 100 ± 3 Ma intrusive suite of Bear Valley crosscuts the older suite, but also exhibits significant synplutonic deformation. Mainly concordant zircon ages indicate the igneous crystallization age, but some discordances occur due to inheritance or entrainment of Proterozoic zircon. The high‐temperature deformation fabrics in these suites and within the metasedimentary framework rocks were crosscut by the granodiorite of Claraville (90 Ma) and pegmatite dikes (∼95 Ma). The granodiorite of Claraville shows strong inheritance of Proterozoic zircon and high initial 87 Sr/ 86 Sr and δ 18 O. Zircon populations from paragneiss and quartzite samples are dominated by Proterozoic detrital grains. Strontium and oxygen isotopic data on the zircon geochronology sample suite suggest simple twocomponent mixing of mantle‐derived gabbroic to tonalitic magmas with partial to complete melt products from the metasedimentary framework rocks. Sedimentary admixtures for some granitic rocks may be as high as 45%, but for the tonalitic batholithic complex are no higher than about 15%. Modeled values of 10–20% metasediment are typical for the orthogneisses. Initial 87 Sr/ 86 Sr correlates directly with δ 18 O, and generally correlates inversely with Sr content. Some subtle complexities in the Sr and O isotopic data suggest the involvement of a third cryptic component. Such a component could be early Phanerozoic ensimatic accretionary terranes that were structurally beneath the observed metasedimentary sequence, or altered oceanic crust and sediments introduced into the mantle magma source area by subduction. One of the initial aims of this study was to seek out remnants of Proterozoic sialic crystalline rocks within the gneiss complex of the Tehachapi Mountains. No such remnants were found, and our studies strongly suggest that sialic components within this link of the Mesozoic batholithic belt were introduced into mantle‐derived magraatic systems by anatexis of continent‐derived sedimentary rocks.
Batholith
Anatexis
Cite
Citations (75)
Research Article| February 01, 1995 Crustal contamination in the petrogenesis of a calc-alkalic rock series: Josephine Mountain intrusion, California Andrew P. Barth; Andrew P. Barth 1Department of Geology, Indiana/Purdue University, Indianapolis, Indiana 46202 Search for other works by this author on: GSW Google Scholar J. L. Wooden; J. L. Wooden 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar R. M. Tosdal; R. M. Tosdal 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. Morrison J. Morrison 3Department of Geological Sciences, University of Southern California, Los Angeles, California 90089 Search for other works by this author on: GSW Google Scholar Author and Article Information Andrew P. Barth 1Department of Geology, Indiana/Purdue University, Indianapolis, Indiana 46202 J. L. Wooden 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 R. M. Tosdal 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 J. Morrison 3Department of Geological Sciences, University of Southern California, Los Angeles, California 90089 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1995) 107 (2): 201–212. https://doi.org/10.1130/0016-7606(1995)107<0201:CCITPO>2.3.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 Andrew P. Barth, J. L. Wooden, R. M. Tosdal, J. Morrison; Crustal contamination in the petrogenesis of a calc-alkalic rock series: Josephine Mountain intrusion, California. GSA Bulletin 1995;; 107 (2): 201–212. doi: https://doi.org/10.1130/0016-7606(1995)107<0201:CCITPO>2.3.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 The Josephine Mountain intrusion is a Cretaceous calc-alkalictonalite-granite pluton emplaced at 22 km depth in a continental margin arc. Variable uplift of adjacent terranes in southern California since mid-Cretaceous time allows us to reconstruct the local crustal column and evaluate its role as a contaminant of mantle-derived arc magmas in this region. The parental magma of the intrusion was high-alumina basalt whose isotopic signature (87Sr/86Sr = 0.7087; δ18O = 7.5; ϵNd = −10) cannot have been generated by intracrustal assimilation of known or inferred rock types in the middle or lower crust. Such a signature could have resulted from high-pressure fractionation of primary low-alumina basalt coupled with assimilation of felsic/pelitic lower crust, partial melting of enriched subcontinental mantle followed by high-pressure fractionation, or a combination of these processes. Tonalite of the intrusion was formed by fractionation of the parent magma coupled with assimilation of local felsic wall rocks or by crustal melts similar to slightly younger granite. Assessment of the magnitude of crustal contamination is hampered by uncertainty regarding the existence and role of partial melting of previously enriched subcontinental mantle in generating the parental basaltic magma, leading to concomitant uncertainty in the fraction of new continental crust created by such arc plutonism. This content is PDF only. Please click on the PDF icon to access. 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.
Geological survey
Petrogenesis
Memphis
Geologic map
Cite
Citations (68)
Strontium and neodymium isotopic data for rocks from the voluminous 90-million-year-old Lamarck intrusive suite in the Sierra Nevada batholith, California, show little variation across a compositional range from gabbro to granite. Data for three different gabbro intrusions within the suite are identical within analytical error and are consistent with derivation from an enriched mantle source. Recognition of local involvement of enriched mantle during generation of the Sierran batholith modifies estimates of crustal growth rates in the United States. These data indicate that parts of the Sierra Nevada batholith may consist almost entirely of juvenile crust added during Cretaceous magmatism.
Batholith
Cite
Citations (84)
Garnet pyroxenites are the most common deep lithospheric xenolith assemblages found in Miocene volcanic rocks that erupted through the central part of the Sierra Nevada batholith. Elemental concentrations and isotope ratios are used to argue that the Sierra Nevada granitoids and the pyroxenite xenoliths are the melts and the residues/cumulates, respectively, resulting from partial melting/fractional crystallization at depths exceeding 35–40 km. The estimated major element chemistry of the protolith resembles a basaltic andesite. Effectively, at more than about 40 km depth, batholith residua are eclogite facies rocks. Radiogenic and oxygen isotope ratios measured on pyroxenites document unambiguously the involvement of Precambrian lithosphere and at least 20–30% (mass) of crustal components. The mass of the residual assemblage was significant, one to two times the mass of the granitic batholith. Dense garnet pyroxenites are prone to foundering in the underlying mantle. An average removal rate of 25–40 km 3 /km Myr is estimated for this Cordilleran‐type arc, although root loss could have taken place at least in part after the cessation of arc magmatism. This rate is matched by the average subcrustal magmatic addition of the arc (∼23–30 km 3 /km Myr), suggesting that the net crustal growth in this continental arc was close to zero. It is also suggested that in order to develop a convectively removable root, an arc must have a granitoid melt thickness of at least 20–25 km. Residues of thinner arcs should be mostly in the granulite facies; they are not gravitationally unstable with respect to the underlying mantle.
Batholith
Fractional crystallization (geology)
Continental arc
Island arc
Back-arc basin
Cite
Citations (156)
Research Article| June 01, 1988 Magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization JAY J. AGUE; JAY J. AGUE 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar GEORGE H. BRIMHALL GEORGE H. BRIMHALL 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Search for other works by this author on: GSW Google Scholar Author and Article Information JAY J. AGUE 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 GEORGE H. BRIMHALL 1Department of Geology and Geophysics, University of California, Berkeley, California 94720 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1988) 100 (6): 912–927. https://doi.org/10.1130/0016-7606(1988)100<0912:MAAADO>2.3.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 JAY J. AGUE, GEORGE H. BRIMHALL; Magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization. GSA Bulletin 1988;; 100 (6): 912–927. doi: https://doi.org/10.1130/0016-7606(1988)100<0912:MAAADO>2.3.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 In order to better understand geologic fac-tors controlling pronounced regional variations in whole-rock chemistry, mineralogy, and mineral chemistry in the batholiths of California, we calculate the magmatic intensive variables fHF/fH2O, fHF/fHCl, and fO2.Regional-scale west-to-east increases in F/OH in mafic silicates, corresponding to the systematic I-WC to I-MC and I-SC progression, reflect orders-of-magnitude increase in fHF/fH2O attending pluton crystallization. Low fHF/ fH2Oof formation of western I-WC types is consistent with their derivation from low-fluorine source rocks in subducted oceanic slabs or the upper mantle. In contrast,higher fHF/fH2Oof crystallization of I-MC and I-SC types to the east implies the involvement of (1) progressively greater amounts of continental crustal source material such as biotite-bearing metamorphic rocks, their unweathered sedimentary derivatives retaining F-rich mafic minerals, or their fusion products and/or (2) source materials which become more F-rich toward the continental interior. The regional distribution of I-MC and I-SC types suggests that the Precambrian craton of western North America, or derivative sediments, may extend farther north in California and be morphologically more complex than previously thought. From seemingly out-of- place occurrences of I-WC plutons on the eastern slopes of the Sierra Nevada batholith, we infer the existence of regions where Precambrian basement was thin or absent in Mesozoic time, which prevented extensive cratonal contamination of subducted slab or upper mantle-derived magmas.New methods for estimating T-fO2 relations in the magmas demonstrate that I-SCR granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of I-WC, I-MC, and I-SC types under conditions at or below the maximum stability limit of graphite in equilibrium with a C-O-H-S gas phase. The local-scale formation of I-SCR granites in plutonic belts within specific wall-rock terranes containing highly reducing sediments or metasediments may occur by contamination of I-types with graphitic pelite or, in some cases, by the direct fusion of this reducing pelitic wall rock. The spatial distribution of I-SCR granite provinces therefore is controlled simply by wall-rock lithology.Amphibole geobarometry demonstrates a general west-to-east decrease in crystallization pressure across the Sierra Nevada batholith. In contrast, the Peninsular Ranges batholith displays a west-to-east crystallization pressure increase. The bulk of the California batholiths crystallized at pressures less than 4-5 kb and depths less than about 15-19 km. In the southern Sierra Nevada batholith, the San Gabriels, and the eastern Peninsular Ranges, however, plutons crystallized at pressures exceeding 6 kb at deep crustal levels (>23 km). Reconstruction of the pre-erosion top of the batholith shows that in an east-west cross section, the central Sierra Nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one. This content is PDF only. Please click on the PDF icon to access. 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.
Batholith
Cite
Citations (313)
Abstract The Late Cretaceous was a period of extremely voluminous magmatism and rapid crustal growth in the western United States. From approximately 98 to 86 Ma, greater than 4000 km2 of exposed granodioritic to granitic crust, including the largest composite intrusive suites in the Sierra Nevada batholith, were emplaced in eastern California. Plutons intruded during this period include the highest peaks in the Sierra; we informally refer to this as the Sierra Crest magmatic event. Field, petrologic, geochemical, and geochronologic data indicate that, although they comprise an insignificant volume of exposed rocks (less than 100 km2), mafic magmas were intruded contemporaneously with each episode of intermediate and high-silica magmatism in the event. This observation attests to the fundamental importance of high-alumina basaltic magmas during crustal-growth episodes in continental arcs. Geochemical data for suites of coeval plutonic rocks of the Sierra Crest magmatic event, ranging in composition from basalt to high-silica rhyolite, demonstrate that recycling of pre-existing crust locally played a minor role in the growth of new crust. Thus, major chemical and isotopic characteristics of Sierra Crest plutons, such as variable isotopic compositions, were inherited from the mantle source of the high-alumina basalts and are not necessarily the result of interaction with the overlying crust. Consequently, we interpret isotopic boundaries in the western United States, such as the 87Sr/86Sr = 0.706 isopleth, to be largely features of the continental lithospheric mantle. Furthermore, isotopic data demonstrate that enrichment of the lithospheric mantle in the western United States probably occurred in the Precambrian during assembly of the North American craton. Geophysical and xenolith investigations by other workers support the hypothesis presented here that Cretaceous magmatism in the Sierra Nevada may have locally restructured most, if not all, of the crustal column. The timing of Sierra Crest magmatism correlates with voluminous magmatism elsewhere in the Cordilleran arc. We speculate that this intense episode of magmatism may have played a role in the global marine geochemical excursions and extinctions at the Cenomanian-Turonian boundary.
Crest
Cite
Citations (160)
Plutonic igneous rocks of the Sierra Nevada batholith exhibit a range of Nd isotopic composition described by ε Nd = +6.5 to −7.6. Similar rock types from the Peninsular Ranges have ε Nd = +8.0 to −6.4. In both batholiths, ε Nd correlates strongly with initial 87 Sr/ 86 Sr. Decreasing ε Nd values are accompanied by increasing 87 Sr/ 86 Sr and increasing δ 18 O; the correlation with δ 18 O being more pronounced for the Peninsular Ranges. The ε Nd values show regular geographic variations, as was found previously for initial 87 Sr/ 86 Sr. Three metasedimentary country rock samples from the Sierra Nevada region have low ε Nd values (−11 to −16) and Precambrian model Sm‐Nd ages (1.5 to 1.9 Æ). The country rock ε Nd values, and those of primitive oceanic island arcs (ε Nd = +8), bracket the data for the batholith rocks. The Nd, Sr, and O isotopic data can be explained if the batholiths are mixtures of island arc and metasedimentary components, the latter being of both Paleozoic and early Proterozoic age. This model appears to be consistent with existing Pb isotopic data. Consideration of O‐Sr isotopic relations and the variation of 147 Sm/ 144 Nd with ε Nd suggests that assimilation of crustal rocks by magmas rising from the mantle and undergoing fractional crystallization could have been the major process responsible for the mixing of crustal‐ and mantle‐derived components. The isotopic data, when combined with assumptions about the structure of the crust beneath the batholiths, suggest that about 50% of the crustal material presently within the geographic boundaries of the batholiths and above the Moho represents juvenile crust derived from the mantle in the Mesozoic. The remaining material appears to be mostly derived from 1.8‐Æ crust, yielding an average crust formation age of nearly 1 Æ for this section of the crust. This result, which may apply to large portions of the Cordillera, suggests that the average age of the North American continent may be greater than previously estimated. The concentration of Nd correlates well with ε Nd in the batholith rocks and supports the conclusion that juvenile continental crust is derived from mantle reservoirs that are depleted in incompatible elements. A 1.5‐Æ Sm‐Nd model age for sedimentary rocks of the Mesozoic(?) Calaveras Formation indicates that the Nd in this “oceanic” terrain is dominated by continental detritus and demonstrates the potential of Sm‐Nd isotopic studies for aiding in construction of tectonic models.
Batholith
Fractional crystallization (geology)
Cite
Citations (901)
This study provides new information on the timing and distribution of Mesozoic magmatic events in the Sierra Nevada batholithic complex chiefly between 36° and 37°N. latitude. U‐Pb ages have been determined for 133 zircon and 7 sphene separates from 82 samples of granitoid rocks. Granitoid rocks in this area range in age from 217 to 80 m.y. Triassic intrusions are restricted to the east side of the batholith; Jurassic plutons occur south of the Triassic plutons east of the Sierra Nevada, as isolated masses within the Cretaceous batholith, and in the western foothills of the range; Cretaceous plutons form a continuous belt along the axis of the batholith and occur as isolated masses east of the Sierra Nevada. No granitic intrusions were emplaced for 37 m.y. east of the Sierra Nevada following the end of Jurassic plutonism. However, following emplacement of the eastern Jurassic granitoids, regional extension produced a fracture system at least 350 km long into which the dominantly mafic, calc‐alkalic Independence dike swarm was intruded 148 m.y. ago. The dike fractures probably represents a period of regional crustal extension caused by a redistribution of the regional stress pattern accompanying the Nevadan orogeny. Intrusion of Cretaceous granitic plutons began in large volume about 120 m.y. ago in the western Sierra Nevada and migrated steadily eastward for 40 m.y. at a rate of 2.7 mm/y. This slow and constant migration indicates remarkably uniform conditions of subduction with perhaps downward migration of parent magma generation or a slight flattening of the subduction zone. Such steady conditions could be necessary for the production of large batholithic complexes such as the Sierra Nevada. The abrupt termination of plutonism 80 m.y. ago may have resulted from an increased rate of convergence of the American and eastern Pacific plates and dramatic flattening of the subduction zone. U‐Pb ages of the Giant Forest‐alaskite sequence in Sequoia National Park are all in the range 99±3 m.y., indicating a relatively short period of emplacement and cooling for this nested group of plutons. U‐Pb ages of a mafic inclusion and its host granodiorite indicate that both were derived from a common source or that the mafic inclusion was totally equilibrated with the granodioritic magma. Comparison of isotopic ages determined by different methods such as zircon U‐Pb, sphene U‐Pb, hornblende K‐Ar, and biotite K‐Ar suggests that zircon U‐Pb ages generally approximate the emplacement age of a pluton. However, some plutons probably contain inherited or entrained old zircons, and the zircons of some samples are disturbed by younger thermal and metamorphic events. The ages reported here are consistent with U‐Pb age determinations previously made on granitic rocks to the north [Stern et al., 1981], The age distribution of granitic belts determined here is in general agreement with those established by K‐Ar dating [Evernden and Kistler, 1970] but does not differentiate the five epochs of plutonism determined in their study.
Batholith
Plutonism
Dike
Geochronology
Orogeny
Cite
Citations (291)