Kenneth L. Cameron

Lu–hf garnet geochronology: closure temperature relative to the Sm–Nd system and the effects of trace mineral inclusions

The highly elevated Lu/Hf of garnets with respect to other minerals, coupled with the new capability of routinely analyzing small samples (25 ng of Hf) by multiple-collector ICP-MS (MC–ICP–MS), makes the Lu–Hf garnet system a viable geochronometer. The robustness of Lu–Hf garnet-whole rock (gt-wr) ages, however, needs to be evaluated, and their closure temperature (TC) and potential effects of trace mineral inclusions need to be established. To constrain the TC of Lu–Hf relative to that of Sm–Nd in gt-wr systems, we used thermal ionization mass spectrometry (TIMS) and MC–ICP–MS techniques to determine the Lu–Hf and Sm–Nd ages of garnet-bearing rocks for which the general thermochronology had been previously established. Samples include the Huiznopala Gneiss (Hidalgo, Mexico), the Gore Mountain amphibolite (New York, USA), a xenolith from the Bearpaw Mountains (Montana, USA), and the Smith Grade Granite (California, USA). In addition to whole rocks and garnet, the Lu–Hf isotope compositions of hornblende, zircon, and monazite were also measured. Our data suggest that the TC of Lu–Hf is greater than or equal to the TC of Sm–Nd in gt-wr systems that cooled slowly (<10°C/m.y.) from granulite facies conditions. There is no single TC for Lu–Hf or even a restricted range of TC that applies to all garnets, as is the case for Sm–Nd. Leaching experiments and trace element modeling show that monazite and apatite inclusions may severely affect the Sm–Nd systematics of garnet, but they have little or no effect on the Lu–Hf system. In contrast, zircon, with its high Hf content, can strongly influence the Lu–Hf systematics of garnets and whole rocks. Zircon from two samples did not achieve Hf isotope equilibrium with the rest of the rock at the time indicated by gt-wr isochrons. Zircon is thus capable of preserving an inherited Hf component through periods of high-grade metamorphism. If present in the matrix only, such zircon will cause erroneously old Lu–Hf ages, while such zircon present only in the garnet will yield ages that are too young. If inherited zircon is distributed evenly throughout the garnet and matrix, the competing age effects will partially cancel, but the age will be too young if the rock contains a phase (e.g., hornblende) that buffers the matrix against the influence of inherited zircon Hf. For rocks that contain inherited zircon, the maximum effects on the gt-wr age must be determined before the age can be interpreted with confidence. Garnets that have significant zircon inclusion contents (i.e., 176Lu/177Hf ≲ 0.3 in this study) should be avoided for Lu–Hf gt-wr geochronology.

Petrogenesis of voluminous mid-Tertiary ignimbrites of the Sierra Madre Occidental, Chihuahua, Mexico

The mid-Tertiary ignimbrites of the Sierra Madre Occidental of western Mexico constitute the largest continuous rhyolitic province in the world. The rhyolites appear to represent part of a continental magmatic arc that was emplaced when an eastward-dipping subduction zone was located beneath western Mexico.In the Batopilas region of the northern Sierra Madre Occidental the mid-Tertiary Upper Volcanic sequence is composed predominantly of rhyolitic ignimbrites, but volumetrically minor lava flows as mafic as basaltic andesite are also present. The basaltic andesite to rhyolite series is calc-alkalic and contains ∼1% K2O at 60% SiO2. Trace element abundances of a typical ignimbrite with 73% SiO2 are Sr ∼ 225 ppm, Rb ∼130 ppm, Y ∼32 ppm, Th ∼12 ppm, Zr ∼200 ppm, and Nb ∼15 ppm. The entire series plots as coherent and continuous trends on variation diagrams involving major and trace elements, and the trends are distinct from those of geographicallyassociated rocks of other suites. We interpret these and other geochemical variations to indicate that the rocks are comagmatic. Mineral chemistry, Sr isotopic data, and REE modelling support this interpretation.Least squares calculations show that the major element variations are consistent with formation of the basaltic andesite to rhyolite series by crystal fractionation of observed phenocryst phases in approximate modal proportions. In addition, calculations modelling the behavior of Sr with the incompatible trace element Th favor a fractional crystallization origin over a crustal anatexis origin for the rock series. The fractionating minerals included plagioclase (> 50%), and lesser amounts of Fe-Ti oxides, pyroxenes, and/or hornblende. The voluminous ignimbrites represent no more than 20% of the original mass of a mantle-derived mafic parental magma.

Contrasting styles of Pre‐Cenozoic and Mid‐Tertiary crustal evolution in northern Mexico: Evidence from deep crustal xenoliths from La olivina

The principal deep crustal rock types found at the La Olivina xenolith locality in southeastern Chihuahua, Mexico, are mafic granulites, paragneisses, and intermediate- to silicic-composition orthogneisses. These granulite facies xenoliths are interpreted in terms of two age groups, pre-Cenozoic and mid-Tertiary, based on previous ion probe dating of zircons from the xenoliths and on isotopic comparisons of the xenoliths to rocks of known age. The mafic granulites have Pb, Nd, and Sr isotopic compositions identical to those of Oligocene volcanic rocks from the La Olivina region. Compositionally, they are olivine-normative gabbroic cumulates, and they precipitated from two or more mid-Tertiary basalt to dacite or rhyolite assimilation/fractional crystallization series. Mineral assemblages in the xenoliths record pressures of ≤7.2 kbar or depths of 35 km thick in Oligocene time as inferred from regional tectonic considerations, then the mafic granulites cannot be samples of basaltic magmas underplated near the crust-mantle boundary. The cumulate protoliths for the mafic granulites probably formed in magma chambers well above the Moho. The mafic granulites are plausibly representative of kilometers of new crust that formed in mid-Tertiary time beneath the extensive ignimbrite fields of Mexico. Most orthogneiss xenoliths are pre-Cenozoic, and they are rocks associated with the late Paleozoic Ouachita Orogeny and older events (e.g., Proterozoic basement and Paleozoic arc rocks). The Ouachita Orogeny was a collision event involving North America and a continental plate to the south, and the results of this study indicate that La Olivina is located above the southern plate. The paragneiss xenoliths overlap in isotopic composition with Carboniferous flysch units exposed in the Marathon uplift of west Texas. These sedimentary rocks and the sedimentary protoliths of the paragneiss xenoliths were derived from the southern plate and deposited before the orogeny in the ocean basin that separated North America and the southern plate. The paragneisses were not metamorphosed to granulite facies until mid-Tertiary time. Pre-Cenozoic and mid-Tertiary crustal evolution followed very different paths in northern Mexico. For example, Nd isotopic evidence for crustal recycling is much more evident in rocks associated with the Paleozoic convergence than in rocks produced during mid-Tertiary magmatism. Furthermore, mafic rocks are very rare in the pre-Cenozoic xenolith suite, but they dominate the mid-Tertiary one. These contrasting characteristics of the pre-Cenozoic and mid-Tertiary xenolith suites are interpreted to reflect differences in tectonic environment and crustal thickness. Preceding the collision event, the southern plate had an Andean-type margin, and the abundant evidence for crustal recycling during this time implies that the crust was thick. The rarity of pre-Cenozoic mafic xenoliths suggests that Proterozoic and Paleozoic lower crust may have delaminated in response to crustal overthickening associated with the Ouachita collision event.

Rare earth element, 87Sr/86Sr, and 143Nd/144Nd compositions of Cenozoic orogenic dacites from Baja California, northwestern Mexico, and adjacent west Texas: evidence for the predominance of a subcrustal component

Dacitic lavas and ignimbrites were examined from seven localities that span the entire 700 km width of the mid- to late Cenozoic magmatic arc of northwestern Mexico and adjacent west Texas. These rocks have remarkably similar REE patterns that are parallel in the heavy REE and have modest negative Eu anomalies. Samples from three localities including Baja California, the Sierra Madre Occidental, and the Chihuahuan Basin and Range have initial 87Sr/86Sr between 0.7044 and 0.7050 and ɛNd near 0.0±1.0. These dacites are isotopically similar to associated basalts, and they show no systematic isotopic variation that is correlated with age or composition of the basement. There is no evidence that magmas parental to these dacites interacted significantly with continental crust. Samples form three other localites in the Basin and Range vary in initial 87Sr/86Sr from 0.7051 to 0.7070 and ɛNd from about -1 to −2. The composition of these rocks reflects contamination of the parental magmas by relatively small amounts of Precambrian crust. Collectively, the dacites of this study show much less isotopic variation than do Mesozoic granitoids (Farmer and DePaolo 1983) and late Cenozoic olivine tholeiites (Hart 1985) from similar transects of the western United States. The distinctive source region for the magmas parental to the Mexican dacites was relatively uniform isotopically, but it was enriched in LIL and HFS elements beneath the eastern Basin and Range.

LuHf geochronology applied to dating Cenozoic events affecting lower crustal xenoliths from Kilbourne Hole, New Mexico

Abstract Melt-extraction or crystal accumulation events that affected garnet-bearing, granulite xenoliths from Kilbourne Hole, New Mexico, have been dated using the LuHf isotope system. Two garnet-bearing granulites from Kilbourne Hole have extreme 176 Lu 177 Hf ratios of 0.95 and 1.3 ( Lu Hf ≈ 28 and 39 × chondritic), but relatively ‘normal’ ϵHf values (+ 5, and + 12) necessitating either garnet accumulation or melt-extraction from a garnet-bearing protolith in the Cenozoic. Hf isotope evolution curves for these two samples intersect those of depleted mantle and Proterozoic crust at high angles and at similar times, demonstrating the potential of Hf isotope model ages to yield true age significance even if the initial Hf isotope composition is not well constrained. A third garnet-granulite xenolith (CKH63; 176 Lu 177 Hf = 0.025 ) contains zircon, which buffered this sample against changes in Lu Hf during the Cenozoic differentiation event. The three garnet granulites lie closely about a 25 Ma LuHf reference line, demonstrating the potential of the LuHf system for detecting garnet-controlled differentiation events in the Cenozoic; given the range in Lu Hf ratios measured, events as young as 5 Ma may be detected using the Lu Hf system. Conventional UPb zircon data from sample CKH63 reveal both a ca. 1.4 Ga inherited component and a component of recent Pb-loss (or new zircon growth), supporting the Cenozoic event documented by Hf isotope model ages.

Rare earth element evidence concerning the origin of voluminous mid-Tertiary rhyolitic ignimbrites and related volcanic rocks, Sierra Madre Occidental, Chihuahua, Mexico

Abstract The mid-Tertiary volcanic sequence of the central Sierra Madre Occidental in Chihuahua, Mexico, is about one kilometer thick and is composed predominantly of rhyolitic ignimbrites. Basaltic andesite to dacitic lavas are interbedded with the rhyolites, but they are of minor volumetric importance. Rare earth element (REE) data are used to constrain a crustal anatexis model for the origin of the voluminous ignimbrites and to test a fractional crystallization model. The REE patterns indicate that if the rhyolites were formed by direct crustal anatexis, the residue from partial melting could contain no more than a few percent garnet or about 20% hornblende. This eliminates residues with the mineralogy of amphibolite, eclogite, or garnet granulite, but melting of a garnet-free granulite source is permitted. The crustal anatexis model is difficult to evaluate critically because of a lack of knowledge concerning the mid-Tertiary geothermal gradient and the composition of the crust beneath the Sierra Madre Occidental. In contrast, the fractional crystallization model can be tested rigorously. Rayleigh fractionation calculations are used to closely model REE patterns in the basaltic andesite to rhyolite series. The minerals involved are those occurring as phenocryst phases, and the mineral proportions were generated by leastsquares major element calculations. The results of the calculations are consistent with the hypothesis that the voluminous rhyolites originated by plagioclase-dominated crystal fractionation.

Whole-rock/groundmass differentiation trends of rare earth elements in high-silica rhyolites

Abstract Whole-rock/groundmass differentiation trends of three series of high-silica rhyolites from northern Mexico and west Texas contrast with that of the Bishop Tuff of California. Together, the four rock series cover a complete spectrum of trends from one where all REE increase with differentiation (peralkaline series of the Chinati Mountains, Texas) to one where all except Ce decrease with fractionation (Batopilas, Mexico). The various differentiation trends reflect different accessory mineral assemblages and different apparent partition coefficient patterns of major mafic phenocrysts. The light REE decrease during differentiation in rock series where mafic phenocrysts and accessory phases are relatively enriched in the light REE ( e.g. , metaluminous series of the Chinati Mountains and the Bishop Tuff). In series that lack light REE-enriched accessory minerals and contain minerals with maxima in their partition coefficient patterns in the middle and heavy REE ( e.g. , zircon, apatite, and hornblende), the middle and heavy REE in the rock series decrease with fractionation ( e.g. , Batopilas). Finally, in rock series that lack abundant REE-enriched accessory minerals and that have relatively small partition coefficients for the major mafic minerals, the REE increase with differentiation ( e.g. , peralkaline series of the Chinati Mountains).

A Late Permian Tectonothermal Event in Grenville Crust of the Southern Maya Terrane: U-Pb Zircon Ages from the Chiapas Massif, Southeastern Mexico

The Chiapas massif (CM) in southeastern Mexico is the crystalline basement of the southern Maya terrane, which is a crustal block that comprises mainly the Yucatan Peninsula, the Mexican states of Chiapas, Veracruz, and parts of Oaxaca. The CM is composed of igneous rocks and mediumto high-grade metamorphic rocks. Zircon fractions from all samples are discordant, yielding Late Permian lower-intercept ages and >1 Ga upper-intercept ages. The most precise results are from an orthogneiss that yielded intercept ages of 258.4 ± 1.9 Ma and 1046.6 ± 5.6 Ma, and from two augen gneisses which yielded intercepts of 250.9 ± 2.3 Ma and 1017 ± 27 Ma. We interpret the lower intercepts as either igneous crystallization or metamorphic ages and the upper intercepts as the age of inherited components. Results from all other samples are within error of these ages. These results demonstrate that the CM basement contains a Grenvillian component, and that the most important tectonothermal event affecting the CM was of Late Permian age. The results favor a hypothetical model in which the Maya terrane is composed of separated blocks of different geologic histories.

Age diversity of the deep crust in northern Mexico

Zircons extracted from five chemically diverse granulite-facies crustal xenoliths from La Olivina cinder cone, Chihuahua, Mexico, were analyzed by SHRIMP ion microprobe in order to determine the age of the deep crust in northern Mexico. The results demonstrate the extreme age diversity of the deep crust in this region: zircon populations of ca. 1, 25-37, ca. 200, 350, 1100, and 1370 Ma are present. Nearly all samples contain 0-20 Ma zircons, reflecting recent granulite-facies conditions in the deep crust. In addition, high-grade metamorphism may also have occurred at 200 Ma and 1.10 Ga (Grenville). The paucity of Carboniferous-Permian ages in the xenoliths suggests that the Ouachita collision event provided little thermal input to the deep crust beneath La Olivina.

Petrologic characteristics of mid-Tertiary volcanic suites, Chihuahua, Mexico

Three suites of mid-Tertiary volcanic rocks are recognized in the state of Chihuahua in northwestern Mexico: (1) a dominantly silicic calc-alkalic suite that includes moderate-K and high-K. facies, (2) a ferroaugite rhyolite suite, and (3) a mildly alkalic basaltic suite. Subduction-related calc-alkalic volcanism was predominant until ∼28 m.y. ago when it was replaced by basaltic volcanism. The source of the basaltic magmas in western Chihuahua was enriched in lithophile elements relative to that of the calc-alkalic magmas. The ferroaugite rhyolites, which are restricted to eastern Chihuahua, appeared ∼32 m.y. ago. They are genetically unrelated to the calc-alkalic suite and are interpreted to represent a westward expansion of alkalic volcanism from west Texas.