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Emerging over the past decade has been a new view on the genesis of, and links between, the Sierra Madre Occidental silicic large igneous province, the Comondu Group of Baja California and the Gulf of California rift. Underpinning this has been a wealth of new data from both margins of the Gulf of California including offshore sampling, and marine geophysical data, in part seeded by the NSF Margins program where the Gulf of California was a principal focus site. Previously, the Sierra Madre Occidental silicic large igneous province and Comondu Group had been widely regarded as supra - subduction volcanism with the Comondu Group in particular, defining the location of the early to mid - Miocene supra - subduction zone volcanic arc, and therefore acting as both a spatial and temporal barrier to when rifting of the Gulf of California could begin. More broadly, this continental magmatism occurring during the last phase of subduction of the Farallon Plate between the Late Eocene and the Middle Miocene, shows little to n o petrogenetic connection to the active plate boundary and is more strongly linked to the progressive thinning of the upper plate and establishment of a shallow asthenospheric mantle beneath western Mexico. A database developed for this study of 4255 ages and chemical analyses for igneous rocks from 100 to 5 Ma from across western Mexico, reveals a significant transition period between 50 and 40 Ma where relatively low - volume magmatism was established across a broad area up to 800 km wide and extended up to 1000 km in board of the paleotrench. Since 40 Ma, magma fluxes greatly increased across this broad belt and compositions were initially silicic - dominated but quickly became bimodal by ~30 Ma. The space - time pattern of crustal extension is constrained in 39 areas, for which the approximate age of extension can be established on the basis of geologic relations or thermochronology. The onset of continental extension is constrained to the Eocene when extensional basins developed across the Central Plateau and the easternmost part of the Sierra Madre Occidental, approximately 500 km in board of the paleo - plate boundary. By the end of Oligocene, crustal extension had affected a wide region (250 km width) from the eastern Sierra Madre Occidental to the site of the future Gulf of California (wide rift mode). Concomitant with this extension was: 1) a widespread invasion of the mid to upper crust by mafic magmas with lithospheric signatures (the southern cordillera orogenic basaltic andesite suite or SCORBA), and lesser erupted volumes of uncontaminated asthenosphere - derived within - plate lavas, and; 2) crustal melting producing voluminous pulses of silicic ignimbrite eruptions (the SMO SLIP) with a ferroan (dry) and transitional within - plate signature. At ~19 Ma, ortho gonal extension became focused between the western side of the SMO and eastern Baja California in a ~80 - 100 km wide belt.

Zircon- and bulk-rock Zr-based thermometric parameters have become fundamental to petrogenetic models of magmatism, from which broader geochronological and tectonic implications are being made. In particular, petrogenetic models have become increasingly reliant on Ti concentration in zircon geothermometry (T ZircTi ) and zircon saturation temperature (T Zircsat ). A feature of many of these studies is an implicit assumption that all zircons present in the host igneous rock are autocrystic, that is, crystallised from the surrounding melt. However, it has long been recognised that zircons present in an igneous rock can be inherited either from the surrounding country rock or source region (xenocrysts), or from earlier phases of magmatism or the magmatic plumbing system (antecrysts). Distinguishing these different origins for zircon crystals or domains within crystals is not straightforward. Here, we first review the utility and reliability of zircon-based thermometers for petrogenetic studies and show that T Zircsat is a theoretical temperature and cannot be used to constrain magmatic or partial melting temperatures. It is a dynamic variable that changes during magma crystallisation, and essentially increases as fractional crystallisation proceeds, whereas true magmatic temperatures (T Magma ) decrease. Generally, in Temperature-SiO2 space, the cross-over point of these two temperatures is magmatic system dependent, and also affected by the type of calibration used for the T Zircsat calculations. Consequently, each magmatic system needs to be evaluated independently to assess the validity and usefulness of T Zircsat . A fundamental conclusion of T Zircsat and T Magma relationships assessed here is that new zircon generally only crystallises in silicic (granitic/rhyolitic) melt compositions, and thus autocrystic zircons should not be assumed to be present in igneous rocks with bulk compositions Zircsat and T Magma ) to estimate whether the magma was zircon-saturated or undersaturated. To test this new protocol, we use as examples, several Phanerozoic granitic rocks intersected by drilling in Queensland where contextual information is limited, and show how antecrystic and xenocrystic zircons and monazites can be distinguished. In contrast, where zircons are metamict (for example, high U and Th-rich zircons), much of the ability to discriminate is impacted because such zircons have suffered Pb loss and have modified compositions (e.g., higher T ZircTi ). We recommend an integrated approach incorporating whole-rock chemistry, independent geothermometric constraints, zircon composition, textures and ages obtained by routine cathodoluminescence and LA-ICP-MS or ion microprobe analysis to provide increased confidence for the discrimination of inherited zircons from autocrystic zircons and determination of the emplacement age.

Granitic magmas extracted from crustal sources can form over a wide variety of P, T and aH 2O conditions. Both fluid-present and effectively fluid-absent conditions can yield granitic magmas, though the majority are formed through high-T, fluid-absent reactions because, in the deep crust, most available H 2O within rock bodies is contained within minerals rather than in free fluids. Fluid-present partial melting generally results in the formation of migmatites (and sometimes diatexites) under upper amphibolite-facies conditions. By definition, aqueous fluid-present melting begins at temperatures very close to the relevant ‘wet’ solidus. Consequently, studying mid-crustal migmatites, and the poorly mobile and mainly small intrusions that may result from such melting, provides no guide to the temperatures and fluid conditions that are involved in the genesis of highly mobile granitic magmas that facilitate crustal differentiation. Although zircon-saturation temperatures are commonly used to infer magma temperatures, and even melting temperatures, they seldom resemble actual magmatic temperatures. Recent notions about the persistence of granite-forming melts at conditions well below the experimentally determined H 2O-saturated (‘wet’) solidi for granites (i.e., generally

Large igneous provinces (LIPs) are sites of the most frequently recurring, largest volume basaltic and silicic eruptions in Earth history. These large-volume (N1000 km3 dense rock equivalent) and large-magnitude (NM8) eruptions produce areally extensive (104–105 km2) basaltic lava flow fields and silicic ignimbrites that are the main building blocks of LIPs. Available information on the largest eruptive units are primarily from the Columbia River and Deccan provinces for the dimensions of flood basalt eruptions, and the Parana–Etendeka and Afro-Arabian provinces for the silicic ignimbrite eruptions. In addition, three large-volume (675– 2000 km3) silicic lava flows have also been mapped out in the Proterozoic Gawler Range province (Australia), an interpreted LIP remnant. Magma volumes of N1000 km3 have also been emplaced as high-level basaltic and rhyolitic sills in LIPs. The data sets indicate comparable eruption magnitudes between the basaltic and silicic eruptions, but due to considerable volumes residing as co-ignimbrite ash deposits, the current volume constraints for the silicic ignimbrite eruptions may be considerably underestimated. Magma composition thus appears to be no barrier to the volume of magma emitted during an individual eruption. Despite this general similarity in magnitude, flood basaltic and silicic eruptions are very different in terms of eruption style, duration, intensity, vent configuration, and emplacement style. Flood basaltic eruptions are dominantly effusive and Hawaiian–Strombolian in style, with magma discharge rates of ~106–108 kg s−1 and eruption durations estimated at years to tens of years that emplace dominantly compound pahoehoe lava flow fields. Effusive and fissural eruptions have also emplaced some large-volume silicic lavas, but discharge rates are unknown, and may be up to an order of magnitude greater than those of flood basalt lava eruptions for emplacement to be on realistic time scales (b10 years). Most silicic eruptions, however, are moderately to highly explosive, producing co-current pyroclastic fountains (rarely Plinian) with discharge rates of 109– 1011 kg s−1 that emplace welded to rheomorphic ignimbrites. At present, durations for the large-magnitude silicic eruptions are unconstrained; at discharge rates of 109 kg s−1, equivalent to the peak of the 1991 Mt Pinatubo eruption, the largest silicic eruptions would take many months to evacuate N5000 km3 of magma. The generally simple deposit structure is more suggestive of short-duration (hours to days) and high intensity (~1011 kg s−1) eruptions, perhaps with hiatuses in some cases. These extreme discharge rates would be facilitated by multiple point, fissure and/or ring fracture venting of magma. Eruption frequencies are much elevated for large-magnitude eruptions of both magma types during LIP-forming episodes. However, in basaltdominated provinces (continental and ocean basin flood basalt provinces, oceanic plateaus, volcanic rifted margins), large magnitude (NM8) basaltic eruptions have much shorter recurrence intervals of 103–104 years, whereas similar magnitude silicic eruptions may have recurrence intervals of up to 105 years. The Parana– Etendeka province was the site of at least nine NM8 silicic eruptions over an ~1 Myr period at ~132 Ma; a similar eruption frequency, although with a fewer number of silicic eruptions is also observed for the Afro- Arabian Province. The huge volumes of basaltic and silicic magma erupted in quick succession during LIP events raises several unresolved issues in terms of locus of magma generation and storage (if any) in the crust prior to eruption, and paths and rates of ascent from magma reservoirs to the surface.

Much has been learned about Large Igneous Provinces (LIPs) and their database greatly expanded since their first formal categorization in the early 1990s. This progress provides an opportunity to review the key characteristics that distinguish LIP events from other melting events of the upper mantle, and to reassess and revise how we define LIPs. A precise definition is important to correctly recognize those LIP events with regional to global effects, and to aid in refining petrogenetic models of the origin of LIPs. We revise the definition of LIPs as follows: “Large Igneous Provinces are magmatic provinces with areal extents > 0.1 Mkm 2 , igneous volumes > 0.1 Mkm 3 and maximum lifespans of ∼ 50 Myr that have intraplate tectonic settings or geochemical affinities, and are characterised by igneous pulse(s) of short duration (∼ 1–5 Myr), during which a large proportion (> 75%) of the total igneous volume has been emplaced.” They are dominantly mafic, but also can have significant ultramafic and silicic components, and some are dominated by silicic magmatism. In this revision, seamounts, seamount groups, submarine ridges and anomalous seafloor crust are no longer considered as LIPs. Although many of these are spatially-related features post-dating a LIP event, they are constructed by long-lived melting anomalies in the mantle at lower emplacement rates, and contrast with the more transient, high magma emplacement rate characteristics of the LIP event. Many LIPs emplaced in both continental and oceanic realms, are split and rifted apart by new ridge spreading centres, which reinforce the link with mid-ocean ridges as a post-LIP event. Three new types of igneous provinces are now included in the LIP inventory, to accommodate the recognition of a greater diversity of igneous compositions, and preserved expressions of LIP events since the Archean: 1) giant diabase/dolerite continental dyke swarm, sill and mafic–ultramafic intrusion-dominated provinces; 2) Silicic LIPs; and 3) tholeiite–komatiite associations, which may be Archean examples of LIPs. A revised global distribution of LIPs for the Phanerozoic is presented. Establishing the full extent of LIPs requires well-constrained plate reconstructions, and at present, plate reconstructions for the Precambrian are poorly known. However, the possibility of reconstructing the LIP record back to and into the Archean and using this expanded LIP record to better constrain the origins and effects of LIPs is an exciting frontier, and our revised definition is a contribution to that effort.