The eastern Transverse Ranges provide essentially continuous exposure for >100 km across the strike of the Mesozoic Cordilleran orogen. Thermobarometric calculations based on hornblende and plagioclase compositions in Mesozoic plutonic rocks show that the first-order distribution of rock units resulted from differential Laramide exhumation. Mesozoic supracrustal rocks are preserved in the relatively little exhumed eastern part of the eastern Transverse Ranges and south-central Mojave Desert, and progressively greater rock uplift and exhumation toward the west exposed rocks originating at mid-crustal depths. The eastern Transverse Ranges thus constitute a tilted, nearly continuously exposed crustal section of the Mesozoic magmatic arc and framework rocks from subvolcanic levels to paleodepths as great as ~22 km. The base of this tilted arc section is a moderately east-dipping sheeted magmatic complex >10 km in width by 70 km in length, constructed structurally beneath, yet synchronous with Late Jurassic and Cretaceous upper-crustal plutons. Geochronology and regional structural relations thus suggest that arc magmas generated in the lower crust of this continental arc interacted in a complex mid-crustal zone of crystallization and mixing; products of this zone were parental magmas that formed relatively homogeneous upper crustal felsic plutons and fed lavas and voluminous ignimbrites.

Abstract This field guide describes a two-and-one-half day transect, from east to west across southern California, from the Colorado River to the San Andreas fault. Recent geochronologic results for rocks along the transect indicate the spatial and temporal relationships between subarc and retroarc shortening and Cordilleran arc magmatism. The transect begins in the Jurassic(?) and Cretaceous Maria retroarc fold-and-thrust belt, and continues westward and structurally downward into the Triassic to Cretaceous magmatic arc. At the deepest structural levels exposed in the southwestern part of the transect, the lower crust of the Mesozoic arc has been replaced during underthrusting by the Maastrichtian and/or Paleocene Orocopia schist.

Most granitic batholiths contain plutons which are composed of low-variance mineral assemblages amenable to quantification of the P – T – f O 2 – f H 2 O conditions that characterise emplacement. Some mineral thermometers, such as those based on two feldspars or two Fe–Ti oxides, commonly undergo subsolidus re-equilibration. Others are more robust, including hornblende–plagioclase, hornblende–clinopyroxene, pyroxene–ilmenite, pyroxene–biotite, garnet–hornblende, muscovite–biotite and garnet–biotite. The quality of their calibration is variable and a major challenge resides in the large range of liquidus to solidus crystallisation temperatures that are incompletely preserved in mineral profiles. Further, the addition of components that affect K d relations between non-ideal solutions remains inadequately understood. Estimation of solidus and near-solidus conditions derived from exchange thermometry often yield results > 700°C and above that expected for crystallisation in the presence of an H 2 O-rich volatile phase. These results suggest that the assumption of crystallisation on an H 2 O-saturated solidus may not be an accurate characterisation of some granitic rocks. Vapour undersaturation and volatile phase composition dramatically affect solidus temperatures. Equilibria including hypersthene–biotite–sanidine–quartz, fayalite–sanidine–biotite, and annite–sanidine–magnetite (ASM) allow estimation of f H 2 O . Estimates by the latter assemblage, however, are highly dependent on f O 2 . Oxygen fugacity varies widely (from two or more log units below the QFM buffer to a few log units below the HM buffer) and can have a strong affect on mafic phase composition. Ilmenite–magnetite, quartz–ulvospinel–ilmenite–fayalite (QUILF), annite–sanidine–magnetite, biotite–almandine–muscovite–magnetite (BAMM), and titanite–magnetite–quartz (TMQ) are equilibria providing a basis for the calculation of f O 2 . Granite barometry plays a critical part in constraining tectonic history. Metaluminous granites offer a range of barometers including ferrosilite–fayalite–quartz, garnet–plagioclase–hornblende–quartz and Al-in-hornblende. The latter barometer remains at the developmental stage, but has potential when the effects of temperature are considered. Likewise, peraluminous granites often contain mineral assemblages that enable pressure determinations, including garnet–biotite–muscovite–plagioclase and muscovite–biotite–alkali feldspar–quartz. Limiting pressures can be obtained from the presence of magmatic epidote and, for low-Ca pegmatites or aplites, the presence of subsolvus versus hypersolvus alkali feldspars. As with all barometers, the influence of temperature, f O 2 , and choice of activity model are critical factors. Foremost is the fact that batholiths are not static features. Mineral compositions imperfectly record conditions acquired during ascent and over a range of temperature and pressure and great care must be taken in properly quantifying intensive parameters.

Abstract Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

The Peninsular Ranges batholith has been subdivided into two zones based on geochemical, geophysical, and lithologic parameters. Plutons in the eastern zone (La Posta–type) are typically larger and inwardly zoned from hornblende-bearing tonalite margins to muscovite-bearing monzogranite cores. U-Pb ages on zircon are generally in the 100 to 90 Ma range. They tend to be more discordant in the cores of the plutons and have upper concordia intercepts near 1300 Ma. Rb-Sr systematics on mineral separates yield an Sr i range from 0.7030 to 0.7044, although one pluton is reported to have a rim-to-core variation from 0.7043 to 0.7074. Whole-rock δ 18 O is lowest in the hornblende-bearing facies (8.3 to 10.9 per mil) and highest in the muscovite-bearing facies (10.2 to 11.8 per mil); the level of variation is pluton dependent. δ 18 O for quartz separates indicate an eastward-directed asymmetry toward heavier oxygen rather than the facies control observed in the whole-rock data. REE patterns from two plutons have nearly identical LREE enrichment and lack any Eu anomaly. Associated with the La Posta–type plutons are a series of small, compositionally restricted, garnet-bearing monzogranites. They are 1 to 5 m.y. younger than the surrounding La Posta–type plutons and contain zircons inherited from a 1200- to 1300-Ma source. Whole-rock δ 18 O values between 12.5 and 13.2 per mil and Sr i = 0.706 reflect a continental contribution to these magmas. La Posta–type melts were generated by subduction-related anatexis of amphibolite-or eclogite-grade oceanic crust. The relatively short emplacement interval and large size of the plutons suggest rapid separation of large volumes of melt from the source region under elevated P H 2 O. Rise toward the present erosional level occurred along the juncture between oceanic lithosphere and the older (ca. 1300 Ma) continental margin. Interaction with the continental crust produced the present-day eastward bias toward higher δ 18 O and zircon discordance.

The Peninsular Ranges of southern and Baja California are divided into a western, predominantly magnetite-bearing plutonic subprovince and an eastern, predominantly magnetite-free plutonic subprovince. The boundary that separates the two subprovinces corresponds roughly to the southwestern margin of the La Posta superunit, but in some places extends into the La Posta granitic province. Neither the pre–La Posta foliated granitic rocks nor the garnet- or muscovite-bearing rocks of the eastern Peninsular Ranges contain magnetite. The magnetite/ilmenite distinction occurs on three scales: regional variations that appear to be independent of host rock or individual plutons, variations paralleling modal facies within zoned plutons, and contact loss of magnetite in the outer margin of a pluton (from meters to more than a kilometer in width). Observations to date indicate that the regional distribution of magnetite- and ilmenite-series granitic rocks may result from generation of parental magma within the dehydration zone of a subduction plane. The gradation within zoned plutons probably results from a lowering of oxygen fugacity in the magma during progressive crystallization. The contact effect appears to be a consequence of reactions between the cooling pluton, the host rocks, and water-rich fluids from a variety of sources.

Mesozoic plutonism in the San Gabriel Mountains has temporal and geochemical similarities to plutonism in the eastern, inboard part of the Sierran–Mojave Desert Mesozoic arc, including the presence of Late Triassic quartz-poor, alkali-enriched plu-tons, Middle Jurassic(?) melanodiorites and porphyritic monzodiorites, a pre–Late Cretaceous bimodal dike swarm, and a Late Cretaceous calc-alkalic suite ranging from metaluminous quartz diorite to peraluminous garnet two-mica granite. Mineralogical evidence demonstrates that these plutonic rocks were emplaced at significantly deeper, mid-crustal levels in comparison to their counterparts on the North American craton. Hornblende barometry in metaluminous rocks yields emplacement depths of 19 to 28 km (5 to 7.5 kbar). The presence of late magmatic epidote in hornblende biotite granodiorite and quartz diorite is consistent with these estimates. Epidote is richer in pistacite component, and thus is indicative of slightly lower pressures of crystallization at higher f O2, in comparison to recent studies of magmatic epidote-bearing plutons. In Late Cretaceous peraluminous rocks, celadonitic muscovite yields estimated emplacement depths of 22 to 33 km (6 to 9 kbar). Calculations indicate that the presence of calcic magmatic garnet in the Late Triassic Mount Lowe intrusion is in accord with these pressure estimates. Mesozoic plutonic rocks in the San Gabriel Mountains are part of an extensive basement terrane (San Gabriel terrane) in southern California, adjacent Arizona, and northern Sonora. The presence of mid-crustal plutonic rocks in the San Gabriel Mountains is geometrically consistent with juxtaposition on the Vincent thrust fault above Pelona Schist, which was metamorphosed at high pressure in a Late Cretaceous–Paleocene subduction event. However, this mid-crustal Mesozoic history appears to characterize only the southern part of the San Gabriel terrane. A northern subterrane of the San Gabriel terrane, containing Proterozoic and Triassic basement rocks correlative with those in the San Gabriel Mountains, had an upper crustal post–Early Jurassic history in close proximity to the North American craton. The mode and timing of juxtaposition of the San Gabriel terrane with autochthonous North America is unclear, but the similarity in histories of Mesozoic arc construction suggests close ties with Mesozoic North America. Much of the distinctive character of the San Gabriel terrane may be a consequence of exposure of rocks formed in the middle crust of an evolving continental margin magmatic arc.

Tectonic decompression of the Whipple Mountains core complex of southeastern California provides a unique window into contrasting crustal levels of magmatic arc development within the Cordilleran orogen. Before uplift of the complex, middle and Late Cretaceous lower plate plutons were emplaced into the crustal section when it resided in the middle crust (depths >27 to 33 km). More shallow emplaced Tertiary plutons, dikes, and sills were intruded as the crustal section was transported upward to a depth of 16 km at approximately 26 Ma, and to less than 6 km by 17 to 19 Ma during final stages of core complex mylonitic and detachment history. Thus, the Whipple crustal section exposes both middle and upper crustal plutonic elements of an evolving magmatic arc. Deep-seated Cretaceous plutons are composed of unusually calcic and Sr-enriched granitic magmas, including two-mica granites that have no compositional counterpart in this region of the Cordillera. Younger plutonism is characterized by more potassic felsic suites and mafic magmas. The oldest plutons compose the 89-Ma Whipple Wash suite, a collection of eight or more granitic intrusions that are marginally metaluminous to moderately peraluminous and include a number of garnet-bearing, two-mica granites. All are low K, high Ca, and high Sr (>800 ppm), and are concluded to have been derived from a high degree of melting of a continental margin, arc-derived, calcic graywacke in equilibrium with an eclogitic residuum. Likewise, primitive members of the high-Sr (>1,000 ppm), 73-Ma Axtel quartz diorite are proposed to have been derived from an eclogitic basalt having an original composition of an altered and/or enriched mid-ocean ridge basalt. Subsequent fractional crystallization (48 percent) of plagioclase, hornblende, and biotite yielded the remainder of the Axtel series. Coeval with core complex mylonitization at 26 Ma, the complex was pervaded by swarms of low-angle dikes and sills of biotite tonalite and trondhjemitic aplite. The tonalite is compositionally restricted and appears to image the same source as that which formed the Whipple Wash suite some 60 m.y. earlier. The aplites form a coherent group of leucocratic and sodic (Na 2 0 to 7.7 wt. %) intrusions that are strikingly depleted in rare earth elements (to subchrondritic values) and other compatible elements. The 10-km-wide Chambers Well dike swarm intruded the western portions of the complex during the Miocene and include an impressive suite of late kinematic andesite to dacite and postkinematic (ca. 21 Ma) diabase. The andesites are inferred to have been derived by partial melting of a partly eclogitized amphibolite; the dacites subsequently evolved by a combined fractionation and crustal assimilation process. The younger diabase dikes represent a separate magma system that formed from limited partial melting of enriched mantle. The last major plutonic event is represented by the 19-Ma War Eagle complex, a composite intrusion consisting largely of clinopyroxene-hornblende diorite and hornblende-biotite quartz diorite. The most primitive rocks are olivine gabbro having a composition consistent with small degrees of partial melting (ca. 5 percent) of a chondritic mantle; the remainder of the complex formed by fractionation of a clinopyroxene, olivine, and plagioclase residue. The War Eagle complex was subsequently intruded by a biotite-hornblende granodiorite, the most potassic and least Sr-rich member of the compositionally evolving assemblage of magmas to be emplaced within the core complex. Most of the plutonic complexes of the inner Cordillera were derived from crustal melting. However, the plutonism in the Whipple core complex is unlike the norm both in composition and inferred origin. Some crustal component is evident in the intermediate to felsic units, yet the inferred sources of calcic graywacke, eclogitic to amphibolitic basalt, and enriched to chondritic mantle signal a subduction setting for the origin of the magmatism.

Structural relief resulting from middle Tertiary extensional deformation in the Chemehuevi Mountains exposes a unique cross section through a temporally and compositionally zoned (both vertically and horizontally), laccolith-shaped intrusion of Late Cretaceous age. The calc-alkalic, metaluminous to peraluminous Chemehuevi Mountains Plutonic Suite exhibits crude normal, vertical, and temporal zonation. The zones are progressively younger and more felsic away from the roof and walls; the most differentiated material is concentrated toward the center and floor of the intrusion. Hornblende-biotite- and biotite granodiorite are metaluminous and form the outer margin of the intrusion along the northern and southern walls, and sill-like bodies in an older suite of granitoids and Proterozoic basement rocks. Locally these rocks bear a sub-horizontal, southwest-trending, mylonitic lineation, considered to be synchronous with regional mylonitic deformation. Later and more evolved units are subequigranular to porphyritic, metaluminous to weakly peraluminous biotite granodiorite to granite, and make up the greatest proportion of the intrusion. The youngest, most leucocratic members of the suite are undeformed, locally garnetiferous muscovite granite and granodiorite that form the central part of the intrusion. Major, trace, and rare earth element data indicate that the magmas of the Cheme-huevi Mountains Plutonic Suite became progressively enriched in Si, K, Rb, Mn, Y, U, and heavy rare earth elements (REE). Fractional crystallization of some REE–rich accessory minerals was important in producing some of these trends. Although modest compositional breaks occur across internal contacts, the general continuity of trends from field, modal, and chemical data suggests that these rocks constitute a comagmatic intrusive suite. Estimates for the pressure of emplacement of the suite vary from 4 to 6 kbar, or a minimum depth of 12 km. Preliminary Pb-, Sr-, and oxygen-isotopic data, together with the REE chemistry, suggest that the Chemehuevi Mountains Plutonic Suite was derived from a heterogeneous crustal source. Compositional variations within the plutonic suite are consistent with open-system fractionation, involving fractional crystallization of discrete batches of magma derived from the melting of a heterogeneous crustal source under H 2 O-saturated conditions.

The Late Cretaceous Old Woman–Piute Range batholith includes both metaluminous and strongly peraluminous granitoid series that intruded the reactivated craton of southeastern California shortly after the orogenic peak. Whole-rock Sr, Nd, and O, feldspar Pb, and zircon U-Pb isotopic compositions, in combination with major- and trace-element and petrographic data, indicate that although these series are not comagmatic, they both were generated primarily by anatexis of Proterozoic crust. Differences between the two rock types are functions of source compositions: peraluminous granitoids were apparently generated from an intermediate to felsic source, metaluminous granitoids from more mafic igneous material with a possible modest subcrustal contribution. No sedimentary input is required in production of the peraluminous granites, and in fact, chemically mature sedimentary material is ruled out as an important contributor— that is, these are not S-type granites. Lead-isotope data reveal that the crust that yielded both magma series had undergone an ancient high-grade uranium depletion event, but independent evidence indicates that at the time of anatexis this crust was by no means anhydrous.

Jurassic plutons in the east-central Mojave Desert region are markedly different from older and younger Mesozoic plutons in the region. They form a chemically and texturally heterogeneous group that ranges in composition from diorite to syenogranite; some phases are alkalic. Igneous rocks in the southern Bristol Mountains, southern Providence Mountains, and Colton Hills are subdivided into five broadly defined groups on the basis of field relations and geochemistry: mafic intrusive rocks; mixed intrusive rocks, consisting of subequigranular and porphyritic subgroups; felsic intrusive rocks; metavolcanic and hypabyssal rocks; and dikes. There is a general trend from older, more mafic and heterogeneous rocks to younger, more felsic and homogeneous plutonic phases. Extreme spatial variations in composition and texture and other field relations indicate that the plutons were intruded during a relatively short time span. Field relations also indicate that the plutons were intruded at upper crustal levels. The plutons were affected by extensive late- or early post-magmatic sodium metasomatism (albitization), which resulted from the introduction and circulation of predominantly meteoric fluids. We propose that the plutons in this region were derived from compositionally heterogeneous but genetically related magmas generated from an upper mantle/lower crustal amphibolitic source. Similar Jurassic rocks are found elsewhere in the southern Cordillera, indicating that the genesis of these unusual rocks is a regional phenomenon.