Testing models of Laramide orogenic initiation by investigation of Late Cretaceous magmatic-tectonic evolution of the central Mojave sector of the California arc

The Mojave Desert region is in a critical position for assessing models of Laramide orogenesis, which is hypothesized to have initiated as one or more seamounts subducted beneath the Cretaceous continental margin. Geochronological and geochemical characteristics of Late Cretaceous magmatic products provide the opportunity to test the validity of Laramide orogenic models. Laramide-aged plutons are exposed along a transect across the Cordilleran Mesozoic magmatic system from Joshua Tree National Park in the Eastern Transverse Ranges eastward into the central Mojave Desert. A transect at latitude ∼33.5°N to 34.5°N includes: (1) the large upper-crustal Late Cretaceous Cadiz Valley batholith, (2) a thick section of Proterozoic to Jurassic host rocks, (3) Late Cretaceous stock to pluton-sized bodies at mesozonal depths, and (4) a Jurassic to Late Cretaceous midcrustal sheeted complex emplaced at ∼20 km depth that transitions into a migmatite complex truncated along the San Andreas fault. This magmatic section is structurally correlative with the Big Bear Lake intrusive suite in the San Bernardino Mountains and similar sheeted rocks recovered in the Cajon Pass Deep Scientific Drillhole. Zircon U-Pb geochronology of 12 samples via secondary ionization mass spectrometry (SIMS) (six from the Cadiz Valley batholith and six from the Cajon Pass Deep Scientific Drillhole) indicates that all Cretaceous igneous units investigated were intruded between 83 and 74 Ma, and Cajon Pass samples include a Jurassic age component. A compilation of new and published SIMS geochronological data demonstrates that voluminous magmatism in the Eastern Transverse Ranges and central Mojave Desert was continuous throughout the period suggested for the intersection and flat-slab subduction of the Shatsky Rise conjugate deep into the interior of western North America. Whole-rock major-element, trace-element, and isotope geochemistry data from samples from a suite of 106 igneous rocks represent the breadth of Late Cretaceous units in the transect. Geochemistry indicates an origin in a subduction environment and intrusion into a crust thick enough to generate residual garnet. The lack of significant deflections of compositional characteristics and isotopic ratios in igneous products through space and time argues against a delamination event prior to 74 Ma. We argue that Late Cretaceous plutonism from the Eastern Transverse Ranges to the central Mojave Desert represents subduction zone arc magmatism that persisted until ca. 74 Ma. This interpretation is inconsistent with the proposed timing of the docking of the Shatsky Rise conjugate with the margin of western North America, particularly models in which the leading edge of the Shatsky Rise was beneath Wyoming at 74 Ma. Alternatively, the timing of cessation of plutonism precedes the timing of the passage of the Hess Rise conjugate beneath western North America at ca. 70–65 Ma. The presence, geochemical composition, and age of arc products in the Eastern Transverse Ranges and central Mojave Desert region must be accounted for in any tectonic model of the transition from Sevier to Laramide orogenesis.Published versio

SHORT MOBILIZATION AND ERUPTION TIMESCALES RECORDED IN THE QUARTZ CRYSTALS OF A FOSSIL CALDERA PLUMBING SYSTEM, SESIA MAGMATIC SYSTEM, SOUTHERN ALPS

Granitic intrusions are not considered the ideal target for the study of short-lived, transient processes associated with remobilization and eruption of highly crystalline silicic magmas. However, crystal zoning preserved in phenocrysts from fossil upper crustal crystal mushes can retain information on the timescales and reactivation dynamics of silicic magma chambers. In the Southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25 Km in tilted crustal blocks. The mid- to upper-crustal segment of this magmatic system (a.k.a. Sesia Magmatic System) is represented by a monzogranitic intrusion ( 4867 to 77 wt% SiO ), the Valle Mosso pluton (VMP), which intrudes cogenetic rhyolitic products of the >15 km diameter Sesia caldera. Field and petrographic evidence suggest that a significant portion of the VMP (ca.15% of the intrusion volume) underwent one or more rejuvenation and mobilization episodes. Titanium (Ti) in quartz content in grains from granitic and volcanic units of the Sesia Magmatic System has been investigated through cathodoluminescence (CL) imaging and microprobe (EPMA) analyses. Sharp contrast in concentration between Ti-poor cores and Ti-rich rims is observed in most of the granitic and volcanic quartz grains. Application of TitaniQ thermometer indicates sharp temperature increase across core-rim boundaries (\u394T of min 50 \ub0C to max 100 \ub0C) assuming uniform a TiO2 and pressure at the time of crystallization. Furthermore, one-dimensional modeling of Ti diffusion core-rime interfaces indicate short elapsed time (10s of years) between crystallization of the high-T rim and cooling of the system below magmatic temperatures, with striking similar results obtained for quartz grains from rejuvenated portion of VMP and volcanic products. These results suggest that a short-lived thermal flare-up, possibly related to mixing with a batch of hotter, more mafic magma, interested the upper portion of the Sesia Magmatic System during its upper crustal residence as a crystal mush, triggering remobilization and eruption of portions of the magma chamber. Such short timescales, typical of explosive eruptive processes, have never been identified before in fossil magma chambers, making this discovery relevant in the framework of the ongoing volcano-plutonic connection debate

Modeling Zircon growth during open-system crystallization

International audienceRecent advances in zircon geochronology by isotope dilution thermal-ionization mass spectrometry (ID-TIMS) techniques allow for age precision beyond the 0.02 % level for single zircon 206Pb/238U dates [1] and can resolve zircon crystallization heterogeneities in a dispersed age population. As zircon saturation is dependent on melt temperature and composition, quantitative analysis of dispersed zircon age populations can provide insight into the timing and magnitude of intensive parameter variations during the lifetime of magma reservoirs

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

In the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution: • Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology. • Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest. • We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories.ISSN:2215-016

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

International audienceIn the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution: • Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology. • Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest. • We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories

A model for thermal rejuvenation and eruption of a granitic magma chamber, Valle Mosso pluton, Sesia Magmatic System (Southern Alps, Italy)

Construction of granitic magma chambers is recognized as a pivotal intermediate stage in the ascent of silicic magmas (SiO in excess of 65 wt%) through continental crust. However, the mechanisms by which large volumes (> 100 km ) of these highly crystalline magmas, stored at mid- to upper-crustal depth, are mobilized and then erupted to the surface in caldera-forming eruptions is still a widely debated issue. In the Sesia Magmatic System, Southern Alps, a Lower Permian pulse of magmatism, which spanned the entire crustal column from a depth of ca. 25 km to the surface, is recorded in a tilted crustal section (Quick et al., 2009). The mid- to upper-crustal segment of this magmatic system is represented by a monzogranitic intrusion ( 48 67 to 77 wt% SiO2), the Valle Mosso pluton (VMP), which intrudes the cogenetic volcanic products (rhyolitic ignimbrites and tuffs) of the now dismembered >15 km diameter Sesia caldera. Field and petrographic evidence suggests that a significant portion of the VMP (ca. 15% of the intrusion volume) underwent one or more rejuvenation and mobilization episodes. The quantitative analyses of disequilibrium textures preserved in quartz, plagioclase and K-feldspar minerals from the rejuvenated portion of the VMP indicate that the mineral-melt disequilibrium is related to reheating of a crystal mush. Evidence includes temperature fluctuations recorded from Ti-in-quartz zoning (650 to >750 \ub0C), dissolution surfaces followed by growth of high-An rims in plagioclase crystals and plagioclase rims on ovoid K-feldspar crystals (rapakivi texture). The presence of andesites both as enclaves in the rejuvenated VMP unit and in the volcanic product of the Sesia caldera hints at an andesitic recharge, which likely induced the thermal perturbation (Quick et al., 2009). New U-Pb zircon dating (SHRIMP) conducted on a sample from the rejuvenated portion of the VMP yields an age of 280.8 \ub1 2.3 Ma, which overlaps with the published 282 Ma age obtained for the main eruptive events in the cogenetic volcanic rocks of the Sesia caldera. On the basis of petrographic, chemical and geochronological evidence a mechanism for the reheating of the VMP is proposed, in which the mixing between a hotter, more mafic magma with a highly crystalline silicic magma has triggered mobilization and eruption of a significant volume of silicic melt

High-precision zircon age spectra record the dynamics and evolution of large open-system silicic magma reservoirs

The emplacement history and thermal evolution of subvolcanic magma reservoirs determine their longevity, size, and ability to feed volcanic eruptions. As zircon saturation is dependent on melt temperature and composition, quantitative analysis of zircon age distributions provides insight into the timing and magnitude of intensive parameter variations during the lifetime of a magma reservoir. Here we present chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon crystallization ages and in-situ zircon trace-element geochemistry for a suite of caldera-related, coeval plutonic and volcanic units of the Permian Sesia Magmatic System (northern Italy). This dataset documents the protracted growth and evolution (∼1 Myr) of a voluminous (>1000 km³), upper crustal (3.5–2.0 kbar) silicic magma reservoir. Systematic changes in zircon composition with time reveal episodic intrusions of new magma into a single, progressively differentiating reservoir which was dominantly kept at high crystallinity conditions (>60 vol.%). The volcanic and plutonic units both show dispersed and heterogeneous CA-ID-TIMS age distributions. A stochastic, thermodynamics-based zircon saturation model accounting for fractional crystallization, recharge and thermal rejuvenation effects on zircon growth and stability in a rhyolitic magma body reproduces the observed distributions of zircon U-Pb ages. Model results suggest that zircon compositional variability and crystallization age heterogeneity, characteristic of the Sesia and other large silicic systems, distinguish open- and closed-system magmatic processes. Specifically, an increase of the mass of crystallized zircons over time is the hallmark of thermally and chemically mature magma reservoirs, capable of producing voluminous eruptions upon changes in the thermal and mechanical state of the system. Conversely, a decrease in crystallized zircon mass over time characterizes systems where crystallization dominates over magma mobility, hindering substantial mass release during eruptive events. We suggest that quantitative analyses of dispersed zircon crystallization age distributions obtained with high-precision techniques can be applied to the plutonic and volcanic record to identify mature silicic magma reservoirs whose properties and storage conditions allowed for catastrophic caldera-forming eruptions. This work provides the petrologic community with a new tool to investigate the genesis and evolution of zircon-bearing intermediate to silicic intrusive magmatism and caldera-forming volcanism through time and across tectonic settings.ISSN:0012-821XISSN:1385-013