1896 marked the beginning of a decade that spawned both modern physics and the science of geochronology based on radioactive decay. The decade started with the discovery of radioactivity by Henri Becquerel in 1896, and ended with the formal publication of ages for natural mineral samples by Ernest Rutherford in 1906. The next fifty years witnessed the discovery of isotopes and nuclear fission; the development of the mass spectrograph and the mass spectrometer; application of the isotope dilution method to dating trace, accessory, and major minerals in typical crustal rocks; and publication of the ca. 4.55 Ga age for the Earth. Yet, after all this, geochronology was still viewed with suspicion by some geologists. In the past fifty years, with additional major advances in instrumentation, technique, and interpretation, geochronology is fully integrated into almost all fields of geology. The three major dating methods from the 1950s and 1960s, U-Pb, K-Ar, and Rb-Sr, have been refined repeatedly. In particular, U-Pb and Ar-Ar, a modern variant of K-Ar, are now capable of <0.1% precisions, with spectacular results in recent studies of crucial problems such as the exact timing and duration of mass extinctions. Many new methods are now available to attack problems ranging from rates of metamorphic mineral growth to rates of uplift and erosion, to the time of surface exposure of geomorphic surfaces. It is a good time to be a geochronologist, or to collaborate with one or more. The future looks very bright.

The composite California Coast Range ophiolite consists of remnants of Middle Jurassic oceanic lithosphere, a Late Jurassic deep-sea volcanopelagic sediment cover, and Late Jurassic intrusive sheets that invade the ophiolite and volcano-pelagic succession. The dismembered Middle Jurassic Coast Range ophiolite remnants (161–168 Ma) were parts of the axial sequence of an oceanic spreading center that consisted of basaltic submarine lava, subvolcanic intrusive sheets, and gabbro, and coeval but off-axis upper lava, dunite-wehrlite mantle transition zone, peridotite restite, and dikes rooted in the mantle transition zone that fed the upper lava. Hydrothermal metamorphism overprints the lavas, subvolcanic sheets, and part of the gabbro. The nearly complete magmatic pseudostratigraphy with minimal syngenetic internal deformation accords with a “hot” thermal structure and robust magma budget, indicative of fast spreading. Upper Jurassic volcanopelagic strata composed of tuffaceous radiolarian mud-stone and chert (volcanopelagic distal facies) overlie the ophiolite lava disconformably and grade up locally into arc-derived deep-marine volcaniclastics (volcanopelagic proximal facies). An ophiolitic breccia unit at northern Coast Range ophiolite localities caps shallow to deep levels of fault-disrupted Middle Jurassic oceanic crust. The Late Jurassic igneous rocks (ca. 152–144 Ma) are mafic to felsic subvolcanic intrusive sheets that invade the Middle Jurassic ophiolite, its Late Jurassic volcanopelagic cover, and locally the ophiolitic breccia unit. Hydrothermal metamorphism of volcanopelagic beds and underlying ophiolite meta-igneous rocks accompanied the Late Jurassic deep-sea magmatic events. The Middle Jurassic ophiolite formed at a spreading ocean ridge (inferred from its Jurassic plate stratigraphy). Intralava sediment and thin volcanopelagic strata atop the Coast Range ophiolite lava record an 11–16 m.y. progression from an open-ocean setting to the distant submarine apron of an active volcanic arc, i.e., the sediments accumulated upon oceanic lithosphere being drawn progressively closer to a subduction zone in front of an ocean-facing arc. Trace-element signatures of Coast Range ophiolite lavas that purportedly link ocean-crust formation to a suprasubduction-zone setting were influenced also by processes controlled by upper-mantle dynamics, especially the mode and depth of melt extraction. The polygenetic geochemical evidence does not decisively determine tectonic setting. Paleomagnetic and biostratigraphic evidence constrains the paleolatitudes of Coast Range ophiolite magmatism and volcanopelagic sedimentation. Primary remanent magnetism in ophiolite lavas at Point Sal and Llanada Coast Range ophiolite remnants records eruption within a few degrees of the Middle Jurassic paleoequator. The volcanopelagic succession at Coast Range ophiolite remnants consistently shows upward progression from Central Tethyan to Southern Boreal radiolarian assemblages, recording Late Jurassic northward plate motion from the warm-water paleo-equatorial realm. Northward seafloor spreading was interrupted by local Late Jurassic rift propagation through the Middle Jurassic oceanic lithosphere. Coast Range ophiolite crust with volcanopelagic soft-sediment cover that lay in the path of propagating rifts hosted rifting-related magmatic intrusions and hydrothermal metamorphism. The advancing broad deformation zone between propagating and failing rifts left paths of pervasive crustal deformation marked now by fault-disrupted ophiolite covered by depression-filling ophiolitic breccias, found at northern Coast Range ophiolite remnants. Coast Range ophiolite lithosphere that lay outside the propagating and failed rift zones lacks those features. The rift-related magmatism and crustal deformation took place at ephemeral spreading-center offsets along a transform fault. Late Jurassic seafloor spreading carried Middle Jurassic oceanic lithosphere northeastward toward a subduction zone in front of the Middle to Late Jurassic arc that fringed southwestern North America. Termination of oblique subduction during the late Kimmeridgian, replaced by dextral transform faulting, left a Coast Range ophiolite plate segment stranded in front (west) of the trench. The trench was then filled and locally bridged by the arc’s submarine sediment apron by the latest Jurassic, allowing coarse volcaniclastic (proximal volcanopelagic) deposits to lap onto earlier, plate-transported tuffaceous radiolarian chert (distal volcanopelagic) deposits. Deep-marine terrigenous muds and sands from southwestern Cordilleran sources then buried the stranded Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit oceanic crust during latest Jurassic northward dextral displacement, which proceeded offshore. Those basal Great Valley Group strata record lower continental-slope and basin-plain marine sedimentation on Jurassic oceanic basement, i.e., the Coast Range ophiolite and adjacent Franciscan oceanic lithosphere (Coast Range serpentinite belt). Forearc basin deposition did not begin until the mid–Early Cretaceous, when the inception of outboard Franciscan subduction lifted and tilted the Coast Range ophiolite–volcanopelagic–ophiolitic breccia unit–basal Great Valley Group succession and Coast Range serpentinite belt to form a basin-bounding forearc ridge. Thereafter, Cretaceous Franciscan subduction and accretionary wedge growth operated in front (west) of the submerged ridge, and Great Valley Group forearc basin terrigenous sediments accumulated behind it.

Geochronology, especially U-Pb zircon geochronology, has made important contributions to our understanding of the Jurassic Coast Range ophiolite of California. However, much of the older work is primitive by modern standards, and even some recent U-Pb work is limited in its precision and accuracy by a range of factors. We apply a new zircon analysis method, chemical abrasion–thermal ionization mass spectrometry (CA-TIMS), to generate high-precision, high-accuracy multistep 206 Pb*/ 238 U plateau ages for zircons from plagiogranites from the Point Sal (Coast Range ophiolite) and San Simeon (Coast Range ophiolite) ophiolite remnants. These remnants have been postulated to have been part of a single, contiguous remnant prior to offset along the San Gregorio–San Simeon–Hosgri fault system. Two fractions of zircon from a Point Sal Coast Range ophiolite plagiogranite, and one fraction of zircon from a San Simeon Coast Range ophiolite plagiogranite yield 206 Pb*/ 238 U plateau ages that are indistinguishable from one another—a mean age for the three determinations is 165.580 ± 0.038 Ma (95% confidence, mean square of weighted deviates [MSWD] = 0.47). The error quoted is an internal precision, which is appropriate for comparison of the ages to one another. The fact that the San Simeon and Point Sal ages are indistinguishable, even with such very small internal precision errors, is a remarkably robust confirmation of the correlation between the San Simeon and Point Sal ophiolite remnants.

La Désirade Island in the Cenozoic Lesser Antilles forearc region exposes a pre-Tertiary complex of oceanic volcanic, plutonic, and dike rocks. Previous work has established the stratigraphy and structure of the La Désirade igneous complex and also its late Mesozoic age. Dredge hauls from the nearby submerged Désirade fault scarp consist of similar volcanic and dike rocks plus greenstone, diabase, and gabbro. The composite section from island and submarine escarpment resembles upper oceanic crust but of controversial origin, original tectonic setting, and geodynamic significance. More precise ages for the La Désirade igneous complex and its individual members provide important constraints on proposed tectonic models. We reanalyzed Radio-laria from intralava sediments in basaltic pillow lava and zircon from trondhjemite to pinpoint their age. The radiolarian assemblage correlates with those of formations in east-central and west-central Mexico. The Mexican radiolarian faunas are chronostratigraphically calibrated by co-occurring ammonites and Buchia. Abundant Mexican biostratigraphic and chronostratigraphic data (ammonites, Radiolaria, and Buchia) constrain the composite radiolarian assemblage from six localities on La Désirade to zone 4, upper subzone 4β (mid-upper Tithonian). Using the new chemical abrasion (CA) thermal ionization mass spectrometry zircon method of Mattinson, the results from three zircon fractions from trondhjemite provide a 143.74 ± 0.33 Ma U-Pb age for the La Désirade igneous complex. Combined biostratigraphic, chronostratigraphic, and geochronometric data put the geochronologic age for the mid-late Tithonian near 143.74 Ma, a maximum for the latest Jurassic.

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Full article available in PDF version

Full article available in PDF version.

The Eagle Rest Peak igneous complex in the San Emigdio Mountains, and Logan gabbro, near San Juan Bautista, California, crop out on opposite sides of the San Andreas fault. They have identical U-Pb zircon ages of 161 Ma and similar Sr initial isotopic ratios. These data support previous correlations of these rocks (Ross, 1970) and require 305 km of post-Jurassic slip on the northern San Andreas fault, a figure equal to the total slip on the southern segment. The Eagle Rest Peak complex has previously been proposed as the source for gabbro clasts in the Upper Cretaceous Gualala Formation near Point Arena (Ross, 1970; Ross and others, 1973). This implies >440 km of post-Late Cretaceous slip on the San Andreas fault and the existence of a proto-San Andreas fault. Also, granitic to quartz dioritic clasts in the Gualala Formation have been interpreted as detritus derived from the Salinian block in the Cretaceous. However, new U-Pb zircon data from Gualala Formation gabbroic clasts indicate minimum ages of 163 and 165 Ma, slightly older than the 161-Ma age of the Eagle Rest Peak complex and Logan gabbro. Published K-Ar ages also suggest the Gualala cobbles are older than the Eagle Rest Peak complex. These data and the presence of alternate sources for the Gualala cobbles indicate that the Gualala-Eagle Rest Peak tie is not suitable for determining slip on the San Andreas fault. Despite the differences between specific areas, the Eagle Rest Peak complex, Gold Hill and Logan gabbros, and the Gualala gabbro clasts are similar in age and lithology to mafic-ultramafic complexes that form a widespread part of the Jurassic Sierran-Klamath arc. Although Gualala gabbro clasts cannot be uniquely matched to the Eagle Rest Peak complex and Logan rocks, they probably are derived from similar rocks cropping out in the Sierra foothills or buried in the Great Valley. U-Pb-age, Sr, and Pb isotopic data from a single Gualala Formation granodiorite clast do not support a Salinian provenance. The clast is older than 154 Ma, older than the 80- to 120-Ma Salinian granites. The clast also has less radiogenic Pb and Sr isotopic ratios than plutonic rocks from the Salinian block. A reevaluation of paleocurrent data and clast types from the Gualala Formation also suggests a non-Salinian source. The Gualala area is apparently not part of the Salinian block.

Granitic rocks ranging in age from Early Cretaceous (about 100 to 110 Ma, and perhaps as old as 120 Ma) to Late Cretaceous (76 Ma) make up the “Salinian magmatic arc” of California. These granitoids, along with a variety of metamorphic country rocks, constitute the pre-Cenozoic basement of the Salinian block or Salinian composite terrane (SCT), an important, but somewhat enigmatic element in the Mesozoic tectonic evolution of the California Cordillera. The ages of magmatic emplacement, based on interpretation of zircon data, reveal a systematic younging to the east across the axis of the arc. The locus of active intrusion migrated eastward at an average rate of about 3 to 4 mm per year, presumably in response to an evolving continental-margin subduction system. Initial Pb, Sr, and Nd isotopic ratios also vary systematically across the arc. The oldest and westernmost intrusions are the most “primitive” in their isotopic characteristics; however, with 87 Sr/ 86 Sr ratios of 0.7055 to 0.7070, ɛ Nd values of −4.4, and fairly radiogenic Pb isotopic ratios, their sources are much more highly evolved or “continental” than the sources of, for example, western Sierra Nevada plutons. Younger SCT plutons, emplaced farther east (and, presumably, farther “inboard”), have even more “continental” signatures, with higher Sr ratios, more negative ɛ Nd values, more “crustal” Pb signatures, and clear evidence for inheritance of Precambrian zircons from their sources. Overall, most of the SCT initial isotopic data appear to define a main mixing trend between more primitive or “oceanic” end members (never observed in the SCT) and a continental crustal end member. The latter may be of metasedimentary origin, perhaps similar to the schist of the Sierra de Salinas, into which some of the plutons intrude. The youngest and easternmost plutons show mixing between the main trend and a distinctive end member with high 208 Pb/ 204 Pb and 87 Sr/ 86 Sr ratios, a low 206 Pb/ 204 Pb ratio, and a low ɛ Nd value. This end member may be the Barrett Ridge gneiss, or similar rocks at depth with a Precambrian history of high-grade metamorphism. Evidently the Salinian magmatic arc straddled the margin of the craton, and the plutons were derived, in part, by anatexis and hybridization of the preexisting basement rocks.

The U-Pb isochron method is a promising new approach to the geochronology of high-pressure-low-temperature metabasites. In samples with favorable U/Pb ratios, metamorphic minerals such as sphene, apatite, lawsonite, glaucophane, garnet, and hornblende partition U and Pb in such a way as to provide a range of U/Pb ratios suitable for isochron dating. In a manner analogous to Rb-Sr isochron dating, these U-Pb isochrons provide not only ages, but also information on the initial isotopic composition of Pb at the time of metamorphism, a significant petrogenetic tracer. Sphene is the key mineral for dating. Its relatively high-U/Pb ratio results in the evolution of moderately radiogenic Pb, and it is highly resistant to resetting. Some metabasites have U/Pb ratios that are extremely low, perhaps owing to severe U depletion at some stage of metamorphism. These samples are not useful for dating, but still provide valuable data on the initial isotopic composition of Pb. Analysis of a Type III metabasalt (blueschist) from the Taliaferro complex near Leech Lake Mountain yields a U-Pb isochron age of 162 ± 3 Ma., slightly older than the widely quoted 150–155 Ma. K-Ar ages for high-grade Franciscan tectonic blocks. Two garnet amphibolite blocks from the Catalina Schist terrane yield identical ages, with an isochron for both samples giving an age of 112.5 ± 1.1 Ma. These samples, plus three more Franciscan metabasites, have isotopic compositions of initial Pb ( 206 Pb/ 204 Pb = 18.40–18.85; 207 Pb/ 204 Pb = 15.55–15.66) that plot distinctly above the field for modern MORB; instead they plot in the fields for some island arcs and granodiorites from the Sierra Nevada.