Douglas M. Morton

Copper Systematics in Arc Magmas and Implications for Crust-Mantle Differentiation

Copper-Bottomed Crust The formation of volcanic arc chains near subduction zones brings large amounts of magma from the upper mantle to the crust, contributing to the formation of island chains in the ocean and adding material to continents. Over time, arc magmas also contribute indirectly to the composition of the oceans and atmosphere through outgassing and weathering of volcanic minerals; however, it is unclear what determines the oxidized nature of arc magmas themselves. Lee et al. (p. 64) measured Cu contents in a range of arc-derived volcanic rocks as a proxy for arc magma redox states. An overall depletion of Cu, which is sensitive to reduced sulfur contents, in global continental crust suggests that there is a hidden reservoir of copper-rich sulfides deep in Earths interior. The copper contents of magmas imply that the formation of sulfide-bearing cumulates under reducing conditions is a critical step in the formation of continental crust. Arc magmas are important building blocks of the continental crust. Because many arc lavas are oxidized, continent formation is thought to be associated with oxidizing conditions. On the basis of copper’s (Cu’s) affinity for reduced sulfur phases, we tracked the redox state of arc magmas from mantle source to emplacement in the crust. Primary arc and mid-ocean ridge basalts have identical Cu contents, indicating that the redox states of primitive arc magmas are indistinguishable from that of mid-ocean ridge basalts. During magmatic differentiation, the Cu content of most arc magmas decreases markedly because of sulfide segregation. Because a similar depletion in Cu characterizes global continental crust, the formation of sulfide-bearing cumulates under reducing conditions may be a critical step in continent formation.

The Mg isotopic systematics of granitoids in continental arcs and implications for the role of chemical weathering in crust formation

Continental crust is too Si-rich and Mg-poor to derive directly from mantle melting, which generates basaltic rather than felsic magmas. Converting basalt to more felsic compositions requires a second step involving Mg loss, which is thought to be dominated by internal igneous differentiation. However, igneous differentiation alone may not be able to generate granites, the most silicic endmember making up the upper continental crust. Here, we show that granites from the eastern Peninsular Ranges Batholith (PRB) in southern California are isotopically heavy in Mg compared with PRB granodiorites and canonical mantle. Specifically, Mg isotopes correlate positively with Si content and O, Sr, and Pb isotopes and negatively with Mg content. The elevated Sr and Pb isotopes require that a component in the source of the granitic magmas to be ancient preexisting crust making up the prebatholithic crustal basement, but the accompanying O and Mg isotope fractionations suggest that this prebatholithic crust preserved a signature of low-temperature alteration. The protolith of this basement rock may have been the residue of chemical weathering, which progressively leached Mg from the residue, leaving the remaining Mg highly fractionated in terms of its isotopic signature. Our observations indicate that ancient continental crust preserves the isotopic signature of compositional modification by chemical weathering.

Regulating continent growth and composition by chemical weathering.

Continents ride high above the ocean floor because they are underlain by thick, low-density, Si-rich, and Mg-poor crust. However, the parental magmas of continents were basaltic, which means they must have lost Mg relative to Si during their maturation into continents. Igneous differentiation followed by lower crustal delamination and chemical weathering followed by subduction recycling are possible solutions, but the relative magnitudes of each process have never been quantitatively constrained because of the lack of appropriate data. Here, we show that the relative contributions of these processes can be obtained by simultaneous examination of Mg and Li (an analog for Mg) on the regional and global scales in arcs, delaminated lower crust, and river waters. At least 20% of Mg is lost from continents by weathering, which translates into >20% of continental mass lost by weathering (40% by delamination). Chemical weathering leaves behind a more Si-rich and Mg-poor crust, which is less dense and hence decreases the probability of crustal recycling by subduction. Net continental growth is thus modulated by chemical weathering and likely influenced by secular changes in weathering mechanisms.

Geophysical and isotopic mapping of preexisting crustal structures that influenced the location and development of the San Jacinto fault zone, southern California

We examine the role of preexisting crustal structure within the Peninsular Ranges batholith on determining the location of the San Jacinto fault zone by analysis of geophysical anomalies and initial strontium ratio data. A 1000-km-long boundary within the Peninsular Ranges batholith, separating relatively mafic, dense, and magnetic rocks of the western Peninsular Ranges batholith from the more felsic, less dense, and weakly magnetic rocks of the eastern Peninsular Ranges batholith, strikes north-northwest toward the San Jacinto fault zone. Modeling of the gravity and magnetic field anomalies caused by this boundary indicates that it extends to depths of at least 20 km. The anomalies do not cross the San Jacinto fault zone, but instead trend northwesterly and coincide with the fault zone. A 75-km-long gradient in initial strontium ratios (Sr i ) in the eastern Peninsular Ranges batholith coincides with the San Jacinto fault zone. Here rocks east of the fault are characterized by Sr i greater than 0.706, indicating a source of largely continental crust, sedimentary materials, or different lithosphere. We argue that the physical property contrast produced by the Peninsular Ranges batholith boundary provided a mechanically favorable path for the San Jacinto fault zone, bypassing the San Gorgonio structural knot as slip was transferred from the San Andreas fault 1.0-1.5 Ma. Two historical M6.7 earthquakes may have nucleated along the Peninsular Ranges batholith discontinuity in San Jacinto Valley, suggesting that Peninsular Ranges batholith crustal structure may continue to affect how strain is accommodated along the San Jacinto fault zone.

Field and model constraints on silicic melt segregation by compaction/hindered settling: The role of water and its effect on latent heat release

Abstract To investigate how large volumes of silicic melts segregate to form granitic plutons, we conducted a case study of a zoned pluton, in which SiO2 increases from intermediate (69 wt%) to highly silicic compositions (74 wt%) toward the contact with metasedimentary wallrock in the outer 25 m of the pluton. All other major, minor, and trace elements vary systematically with SiO2 and indicate that outward increasing SiO2 is due to a decrease in mafic elements and minerals. Whole-rock oxygen isotopes and elemental variation diagrams do not support mixing with wallrock as an explanation for the Si-rich boundary layer. Instead, mafic enclaves, which are common in the pluton, also decrease in abundance in the outer 25 m of the pluton, suggesting a mechanical origin for the Si-rich boundary layer. The coupling of mechanical and geochemical boundary layers, combined with geochemical modeling, indicate that the silica-rich, enclave-poor boundary layer formed by hindered settling or compaction of a crystal-rich (crystal fractions >60%) magmatic mush. Segregation of melts at high crystal fraction is known to be a slow process. However, petrography and Zr-based thermometry indicate that the residual Si-rich liquids were water-saturated. Water decreases melt viscosity, which helps expulsion, but equally importantly, water also delays much of the latent heat release to late in the thermal and crystallization history of a cooling magma. We show that the higher the water content, the longer the time interval over which a magma chamber resides at the stage when water-saturated, high-silica liquids form, allowing sufficient time for exfiltration of silicic liquids before the magma body freezes.

Geophysical evidence for wedging in the San Gorgonio Pass structural knot, southern San Andreas fault zone, southern California

Geophysical data and surface geology define intertonguing thrust wedges that form the upper crust in the San Gorgonio Pass region. This picture serves as the basis for inferring past fault movements within the San Andreas system, which are fundamental to understanding the tectonic evolution of the San Gorgonio Pass region. Interpretation of gravity data indicates that sedimentary rocks have been thrust at least 5 km in the central part of San Gorgonio Pass beneath basement rocks of the southeast San Bernardino Mountains. Subtle, long-wavelength magnetic anomalies indicate that a magnetic body extends in the subsurface north of San Gorgonio Pass and south under Peninsular Ranges basement, and has a southern edge that is roughly parallel to, but 5–6 km south of, the surface trace of the Banning fault. This deep magnetic body is composed either of upper-plate rocks of San Gabriel Mountains basement or rocks of San Bernardino Mountains basement or both. We suggest that transpression across the San Gorgonio Pass region drove a wedge of Peninsular Ranges basement and its overlying sedimentary cover northward into the San Bernardino Mountains during the Neogene, offsetting the Banning fault at shallow depth. Average rates of convergence implied by this offset are broadly consistent with estimates of convergence from other geologic and geodetic data. Seismicity suggests a deeper detachment surface beneath the deep magnetic body. This interpretation suggests that the fault mapped at the surface evolved not only in map but also in cross-sectional view. Given the multilayered nature of deformation, it is unlikely that the San Andreas fault will rupture cleanly through the complex structures in San Gorgonio Pass.

Timing and significance of gabbro emplacement within two distinct plutonic domains of the Peninsular Ranges batholith, southern and Baja California

Evaluating the crucial role postulated for mantle-derived mafic magmas in the formation of silicic batholiths requires well-determined igneous crystallization ages from mafic rocks as well as petrologic data reflecting possible mantle source heterogeneities and/or variations in subcrustal processes. The abundant mafic plutons that crop out within the mid-Cretaceous Peninsular Ranges batholith (PRB, southern and Baja California) provide an important opportunity to investigate the relationship of mafic and silicic magmatism in Cordilleran batholiths. Zircon U-Pb laser ablation inductively coupled plasma mass spectrometry ages are reported for 24 samples of PRB gabbro collected throughout its 1400-km-long extent. The samples span the entire compositional spectrum of PRB gabbro from cumulate olivine gabbro with <10 ppm whole rock Zr content, to gabbronorite, to more fractionated hornblende gabbro. Eighteen of the samples are from the western zone “gabbro belt” of the batholith including the type locality for San Marcos Gabbro, while six are from the eastern zone where gabbro is rare, composing <1% of the outcrop area. The morphology of zircon in PRB gabbro ranges from anhedral fragments to euhedral grains, and this zircon has distinctly lower uranium concentrations and higher Th/U ratios than zircon from PRB granitoids. Most samples exhibit simple U-Pb systematics, but several contain inherited or entrained zircons, and two eastern gabbro samples contain distinctly bimodal age populations. Weighted mean 206Pb/238U zircon ages are interpreted as crystallization ages and range from 130.5 ± 1.5 Ma to 100.2 ± 3.0 Ma for western zone samples. The ages demonstrate that gabbros were emplaced throughout the ∼30 m.y. interval during which the bulk of the western zone was constructed, and conflict with the earlier interpretation of western gabbros as mafic forerunners that preceded emplacement of granitoid plutons. Eastern zone gabbros, in contrast, yield U-Pb ages that range mostly from 109.4 ± 2.9 Ma to 97.8 ± 2.3 Ma, narrowly predating the ca. 92–98 Ma eastern zone tonalite-trondhjemite-granodiorite batholith flareup, and thus may have played a role in crustal thickening and triggering of the flareup. Despite their partially cumulate character, whole rock major and trace element compositions of PRB gabbro closely match high-alumina olivine tholeiite and arc basalt and basaltic andesite from modern arcs. Gabbros from the west and east sides of the batholith, transverse to regional strike, have similar nearly flat rare earth element patterns that reflect derivation from a common partially contaminated depleted mantle source. The apparent across-strike petrologic uniformity of the gabbros contrasts strongly with PRB granitoids, which are well known for across-strike asymmetries in trace elements, initial 87Sr/86Sr, and δ18O, reflecting progressively deeper partial melting of mafic source rocks from west to east with additional input from a high-18O, high-87Sr end member on the east side of the batholith. The western PRB gabbros represent a plausible deep crustal source composition for derivation of western granitoids by partial melting or fractional crystallization. For the eastern batholith, the mafic source region requires modification by input of accretionary complex supracrustal rocks delivered via forearc subduction erosion underthrusting.

Geochemical diagnostics of metasedimentary dark enclaves: a case study from the Peninsular Ranges Batholith, southern California

Dark enclaves rich in amphibole and biotite are ubiquitous in granitoid rocks and typically represent fragments of mafic magmas, cumulates, restites, or country rocks. To develop criteria for identifying dark enclaves of non-magmatic origin, we investigated dark enclaves from a complete spectrum of light (carbonate- or feldspar-rich) to dark (amphibole-rich, biotite-rich, or composite) enclaves, reflecting progressive thermal and chemical equilibration with the host tonalite, the Domenigoni Valley pluton in the Peninsular Ranges Batholith, California. Metasedimentary dark enclaves have geochemical characteristics that overlap those of literature-compiled igneous dark enclaves. When compared with modelled igneous differentiation paths, metasedimentary enclaves can have anomalous CaO and K2O contents for a given SiO2, but other major-element systematics may not deviate noticeably from igneous differentiation trends. In addition, the fact that literature-compiled mafic enclaves trend towards high K2O + CaO suggests that not all mafic enclaves are of igneous origin. In this work, we provide criteria for identifying enclaves of possible metasedimentary origin.