Leander Franz

Magmatic Underplating, Extension, and Crustal Reequilibration: Insights From A Cross‐Section Through the Ivrea Zone and Strona‐Ceneri Zone, Northern Italy

The thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the Ivrea‐Verbano Zone and Strona‐Ceneri Zone in northern Italy. Geochronological and petrologic data are compared to a two‐dimensional thermal‐kinematic model. Field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite‐ to amphibolite‐facies metamorphism as well as reequilibration of mineral chemical and isotopic systems. Magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution, as information on the earlier history is preserved only at upper crustal levels.

The North American-Caribbean Plate boundary in Mexico-Guatemala-Honduras

Abstract New structural, geochronological, and petrological data highlight which crustal sections of the North American–Caribbean Plate boundary in Guatemala and Honduras accommodated the large-scale sinistral offset. We develop the chronological and kinematic framework for these interactions and test for Palaeozoic to Recent geological correlations among the Maya Block, the Chortís Block, and the terranes of southern Mexico and the northern Caribbean. Our principal findings relate to how the North American–Caribbean Plate boundary partitioned deformation; whereas the southern Maya Block and the southern Chortís Block record the Late Cretaceous–Early Cenozoic collision and eastward sinistral translation of the Greater Antilles arc, the northern Chortís Block preserves evidence for northward stepping of the plate boundary with the translation of this block to its present position since the Late Eocene. Collision and translation are recorded in the ophiolite and subduction–accretion complex (North El Tambor complex), the continental margin (Rabinal and Chuacús complexes), and the Laramide foreland fold–thrust belt of the Maya Block as well as the overriding Greater Antilles arc complex. The Las Ovejas complex of the northern Chortís Block contains a significant part of the history of the eastward migration of the Chortís Block; it constitutes the southern part of the arc that facilitated the breakaway of the Chortís Block from the Xolapa complex of southern Mexico. While the Late Cretaceous collision is spectacularly sinistral transpressional, the Eocene–Recent translation of the Chortís Block is by sinistral wrenching with transtensional and transpressional episodes. Our reconstruction of the Late Mesozoic–Cenozoic evolution of the North American–Caribbean Plate boundary identified Proterozoic to Mesozoic connections among the southern Maya Block, the Chortís Block, and the terranes of southern Mexico: (i) in the Early–Middle Palaeozoic, the Acatlán complex of the southern Mexican Mixteca terrane, the Rabinal complex of the southern Maya Block, the Chuacús complex, and the Chortís Block were part of the Taconic–Acadian orogen along the northern margin of South America; (ii) after final amalgamation of Pangaea, an arc developed along its western margin, causing magmatism and regional amphibolite–facies metamorphism in southern Mexico, the Maya Block (including Rabinal complex), the Chuacús complex and the Chortís Block. The separation of North and South America also rifted the Chortís Block from southern Mexico. Rifting ultimately resulted in the formation of the Late Jurassic–Early Cretaceous oceanic crust of the South El Tambor complex; rifting and spreading terminated before the Hauterivian (c. 135 Ma). Remnants of the southwestern Mexican Guerrero complex, which also rifted from southern Mexico, remain in the Chortís Block (Sanarate complex); these complexes share Jurassic metamorphism. The South El Tambor subduction–accretion complex was emplaced onto the Chortís Block probably in the late Early Cretaceous and the Chortís Block collided with southern Mexico. Related arc magmatism and high-T/low-P metamorphism (Taxco–Viejo–Xolapa arc) of the Mixteca terrane spans all of southern Mexico. The Chortís Block shows continuous Early Cretaceous–Recent arc magmatism.

Micro-XANES determination of ferric iron and its application in thermobarometry

Micro-X-ray absorption near-edge structure (XANES) analysis was employed to determine the content of ferric iron in minerals formed in ultrahigh-pressure (UHP) eclogites. It is observed that omphacite and phengite contain significant amounts of Fe3+/Fetot (0.2–0.6), whereas only very low contents are present in garnet (Fe3+/Fetot=0.0–0.03), the latter being consistent with results from stoichiometric charge-balance calculations. Furthermore, considerable variations in the Fe3+/Fetot ratios of omphacite and phengite are observed depending on the textural sites and local bulk chemistry (eclogite and calc-silicate matrix) within one thin section. The oxidation state of isofacial minerals is thus likely to depend on the local fluid composition, which, in the studied case, is controlled by calcareous and meta-basic mineral compositions. These first in-situ measurements of ferric iron in an eclogite sample from the Dabie Shan, E China, are used to recalculate geothermobarometric data. Calculations demonstrate that the temperature during UHP metamorphism was as high as 780 °C, about 80–100 °C higher than previously estimated. Temperatures based on charge balance calculations often give erroneous results. Pressure estimates are in good agreement with former results and confirm metamorphism in the stability field of diamond (43.7 kbar at 750 °C). These P–T data result in a geothermal gradient of ca. 6 °C/km during UHP metamorphism in the Dabie Shan. However, accounting for ferric iron contents in geothermobarometry creates new difficulties inasmuch as calibrations of geothermometers may not be correctable for Fe3+ and the actual effect on Mg–Fe2+ partitioning is unknown. The present study further shows that micro-XANES is a promising technique for the in situ determination of ferric iron contents without destroying the textural context of the sample: a clear advantage compared to bulk methods.

Potassic igneous rocks from the vicinity of epithermal gold mineralization, Lihir Island, Papua New Guinea

Many world-class porphyry copper–gold and epithermal gold deposits worldwide are hosted by volatile-rich and oxidized alkaline rocks. This study investigates potassic igneous rocks from the vicinity of epithermal gold mineralization at Lihir Island, Papua New Guinea. The island consists of five Pliocene–Pleistocene stratovolcanoes, one of which hosts Ladolam, one of the largest epithermal gold deposits discovered to date. Petrographically, the rocks range from porphyritic trachybasalts, trachyandesites and latites to rare phonolites and olivine–clinopyroxene cumulates. In some places, these rocks are cut by monzodiorite stocks. According to Al-in-hornblende barometry, the main crystallization of these rocks occurred close to the surface. Titanium-in-hornblende thermometry as well as olivine–spinel geothermometry and oxygen barometry indicate temperatures of 787–965°C at elevated oxygen fugacities (fO2) of 1.4–4.8 log units above that of the FMQ buffer. Although previous studies have suggested high fO2 of alkaline rocks associated with copper–gold mineralization based on abundant primary magnetite contents, this is the first direct determination of the fO2 of such rocks. High fO2 of parental melts commonly delays the early crystallization of magmatic sulphides; this is important because metals such as Au and Cu preferentially partition into sulphide phases resulting in their depletion in the melt during increasing fractionation. Geochemically, the rocks range from primitive to relatively evolved compositions, as reflected by their SiO2 (45.8–55.0 wt.%) and MgO (1.4–15.3 wt.%) contents and variable concentrations of mantle-compatible elements (130–328 ppm V, 1–186 ppm Ni). Their high K2O content (up to 4.7 wt.%), high average K2O/Na2O ratios (0.8) and high average Ce/Yb ratios (14) are typical of high-K igneous rocks transitional to shoshonites. Although these rocks formed by decompression melting related to back-arc rifting in the Manus Basin, the high LILE, low LREE and very low HFSE concentrations are typical of potassic igneous rocks from oceanic (island) arc settings. The reason for this remarkable composition is the partial melting of subduction-modified lithospheric mantle, which developed in a stalled subduction zone. Mica phenocrysts in the rocks reveal unusually high halogen concentrations. Magmatic phlogopites contain high F (up to 5.6 wt.%) and elevated Cl contents (<0.08 wt.%). Hydrothermal biotites from rocks that display potassic alteration have low F (<0.08 wt.%), but very high Cl concentrations (up to 0.15 wt.%). It is suggested that chloride complexing largely controlled the abundances of Au and Cu in the aqueous fluids responsible for the hydrothermal gold mineralization at Ladolam.

High‐Grade K‐Feldspar Veining in Granulites From the Ivrea‐Verbano Zone, Northern Italy: Fluid Flow in the Lower Crust and Implications For Granulite Facies Genesis

We present observations of extensive networks of K‐feldspar microveins associated with quartz and plagioclase in granulite facies rocks from the Ivrea‐Verbano Zone, northern Italy. Back‐scattered electron images of the granulite facies samples show a continuous system of K‐feldspar veins along quartz/plagioclase and plagioclase/plagioclase grain boundaries that can be traced across the entire length of a thin section. Quartz grain rims in contact with these veins show evidence of extensive corrosion. In contrast, contact between the veins and the orthopyroxene and garnet grains is very clean, with no signs of secondary alteration. K‐feldspar + quartz ± plagioclase, of the same compositions as the veins, are commonly found as near‐rim inclusions within garnet. Microprobe traverses from the interiors of the K‐feldspar veins to their rims show increasing Ba, indicating a metasomatic origin. Some veins display a boudinaged texture, possibly related to the D2 deformation event in the Ivrea‐Verbano Zone and therefore related to the emplacement of the mafic underplating responsible for initiating granulite facies metamorphism. The K‐feldspar also exists as patches of variable size in plagioclase (5–50% of the grain), superficially resembling exsolution antiperthite. The composition of the K‐feldspar “lamellae” in the antiperthite is close to that of the veins, especially with regard to the Ba content. Approximately half of the plagioclase grains contain little K‐feldspar. Remarkably, the K‐feldspar veins and patches diminish in both scope and size with decreasing metamorphic grade and eventually disappear once amphibolite facies is reached. We postulate that the K‐feldspar veins in the Ivrea‐Verbano Zone granulites are evidence of pervasive, high‐temperature, low‐H2O activity brines and that these brines were an important dehydration mechanism from amphibolite grade to granulite grade. Alkali‐ and volatile‐rich mafic underplating, represented today by the basal Mafic Formation in the Ivrea‐Verbano Zone, is the most likely source of the heat and fluids responsible for the genesis of these granulites. This mechanism may have application to other granulite facies terranes where similar K‐feldspar microveining has been observed.

Reequilibration of Ultramafic Xenoliths from Namibia by Metasomatic Processes at the Mantle Boundary

Mantle xenoliths from the Hanaus and the Anis Kubub pipes in the Gibeon Kimberlite Province of southern Namibia show evidence for intensive heating in lower as well as intermediate lithospheric levels. The investigated samples are garnet- and spinel-bearing peridotites (as well as one orthopyroxenite) with granular, partly sheared, and sheared textures. While granular and partly sheared xenoliths reveal distinct zonation patterns, most of the sheared xenoliths display perfect mineral equilibria. Thermobarometric estimates for the primary mineral assemblages of these samples plot in the graphite stability field with maximum P-T conditions of about 1320°C at 44 kbar in garnet harzburgites and a minimum of about 630°C at 19 kbar in spinel lherzolites. The initial steady-state geotherm of

Shallow Drilling of Seafloor Hydrothermal Systems Using the BGS Rockdrill: Conical Seamount (New Ireland Fore-Arc) and PACMANUS (Eastern Manus Basin), Papua New Guinea

44 mW/m^{2}

The effect of water activity on calculated phase equilibria and garnet isopleth thermobarometry of granulites, with particular reference to Tongbai (east-central China)

is only preserved in a few granular xenoliths, whereas the sheared samples plot along an elevated geothermal gradient of about