Slab breakoff: A model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens

http://www.geology.wisc.edu/~johnf/g777/Misc/Montel-1996.pdf

Electron microprobe dating of monazite

Slab breakoff is the buoyancy-driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision. In a recent paper Davies and von Blanckenburg [1994] have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts. Breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere, melting of its enriched layers, and thus to bimodal magmatism. Breakoff will also lead to thermal weakening of the subducted crustal lithosphere, thereby allowing buoyant rise of released crustal slices from mantle depths. In this paper we present a test of this model in the Tertiary evolution of the European Alps. In the Alps, both basaltic and granitoid magmatism occur between 42 and 25 Ma, following the closure of oceanic basins by subduction and continental collision. The granitoids are now well established to result from mixing of basalt with assimilated continental crust. To identify the tectonically crucial origin of the partial mantle melts, we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole Alpine arc. Their trace element and isotopic composition suggests that they have been formed by low-degree melting of the mechanically stable lithospheric mantle. We see no evidence for melting of asthenospheric mantle. It was thus not decompressed to depths shallower than 50 km. Once initiated, rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust. This explains why all Alpine magmatic rocks intruded almost synchronously along a strike-slip fault, the Periadriatic Lineament. A compilation of ages from Penninic high-pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55–40 Ma, followed by uplift at 40–35 Ma. From the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff. The slab breakoff model fulfills its predictions in the case of the Alps and therefore supports the assumptions made in the theoretical model on a geological basis. We believe that the characteristic association of magmatic activity with the return of high-pressure rocks to the surface allows the identification of this process in the Earth's mountain belts.

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Slab breakoff: A model for syncollisional magmatism and tectonics in the Alps

96 new fission track (FT) apatite and zircon, K/Ar and Rb/Sr biotite and muscovite ages are presented for 19 samples (mainly acid gneisses) from a 40 km traverse through the Lepontine Alps in the Maggia Valley, South Central Switzerland. Plotting measured mineral ages against assumed system closure temperatures yields cooling rates for each sample. The entire profile shows a fairly uniform Late Neogene-Recent mean uplift rate of ∼0.5 mm/a, confirmed by a gradient of FT apatite age with elevation. Cooling from higher temperatures occurred earlier in the south, where uplift rates of ∼2.2 mm/a in the Steep Belt (root zone) indicate >9 km Early Miocene uplift of the northern Pennine block. This uplift started before 23 Ma and is interpreted as resulting from a major phase of backthrusting along the Insubric Line, and as dating the formation of the mylonite belt. Estimated cooling rates constrain the timing of Lepontine Mid-Tertiary metamorphism: 3 schematic models are proposed which also consider published Rb/Sr, K/Ar mica and hornblende and U/Pb monazite ages. Slow cooling, differential initial heating and subsequent cooling of different parts of the Central Alps and post-38 Ma cooling with syntectonic metamorphism at ∼27 Ma are postulated as alternative interpretations of isotopic data and geologic evidence. From extrapolation between K/Ar and Rb/Sr mica ages and apatite FT ages, 240±50° C is proposed as the closure temperature for the retention of fission tracks in zircon.

Cooling and uplift patterns in the Lepontine Alps South Central Switzerland and an age of vertical movement on the Insubric fault line

A technique has been developed and tested to analyse 207Pb/206Pb apparent ages by thermal evaporation of radiogenic lead directly from untreated whole zircon grains (≤0.3 mm). The evaporation analyses are performed in the double-filament arrangement of a thermal ion mass spectrometer (ThIMS). The method is a powerful tool to distinguish between different lead components occurring in the same grain because differing activation energies of the competing lead components cause their sequential evaporation from the zircons. The evaporation of test samples results in 207Pb/206Pb apparent ages in good agreement with U/Pb ages known from literature: single zircons from a granite of the ‘Marble Mountains’/California yield an age of crystallization of 1,410±30 Ma; ‘Ceylon’ zircons from heavy-mineral bearing gravels yield 560± 40 Ma as age of crystallization of the pegmatitic gravel sources; individuals from a heterogeneous zircon population of a diatexite from the Southern Schwarzwald/SW-Germany indicate metamorphic zircon formation around 500 Ma and the existence of Middle-Proterozoic relics (1.95±0.05 Ga).

Whole-grain evaporation for 207Pb/206Pb-age-investigations on single zircons using a double-filament thermal ion source

Among the Periadriatic Tertiary calc-alkaline intrusions in northern Italy, the Adamello batholith displays the most convincing evidence for a mafic parentage This is particularly the case for the southern Adamello, where minor ultramafic to gabbroic bodies form the margins of several distinct and dominantly tonalitic plutons Isotopic analyses are presented for the Val Fredda and Blumone ultramafic-gabbroic to tonalitic-leucoquartzdioritic suites, for the more felsic rock types of the spatially predominant Re di Castello group, as well as for a lamprophyre This dike of picrobasaltic composition is thought to represent the parental magma Field relations and a large body of petrographic, geochemical and mineral chemical data point to a cogenetic origin for all of these rocks and to magmatic differentiation with fractional crystallization and accumulation as the main evolutionary mechanism This is compatible with the isotopic results for the Blumone series and the mafic rocks of Val Fredda with eNd = +38 to +15 and eSr = −91 to −46 A common parental magma similar to the lamprophyre (eNd = +31, eSr = −44) also seems plausible The isotopically homogeneous felsic lithologies of Val Fredda (eNd = +01 to −03, eSr = +29 to +07), structurally intimately associated with their mafic counterpart, indicate crustal contamination incurred prior to their differentiation For the Re di Castello group, progressing crustal affinity of isotopic compositions (eNd of −19 to −49 and eSr of +165 to +409) with chemical differentiation implies large-scale interaction with a granitoid crustal component Attempts at quantifying this process reveal individual characteristics and evolution for each pluton The data indicate extraction of this component from a source in the lower crust with time-integrated low Rb/Sr, eSr approximating +50 and eN close to −6 corresponding to a 1000 Ma crustal residence age If a two-stage evolution is assumed, formation of a gabbroic reservoir from the above parental melt would require uptake of some 4–12 wt % of granitoid component by combined assimilation/fractional crystallization, whereas subsequent evolution to tonalitic compositions would necessitate admixing of 40–70 wt % of granitoid partial melt Assimilation thus mainly occurs during evolution from gabbroic to tonalitic compositions, when the mafic magma is ponded near the mantle-crust boundary and where homogenization prior to resuming buoyant ascent is assured

Precise locations and more detailed petrography of the samples is available with entire article on microfiche Order from American Geophysical Union, 2000 Florida Avenue, NW, Washington, DC 20009 Document 91B-002; $250 Payment must accompany order

Nd-Sr isotopic and geochemical characteristics of the southern Adamello (northern Italy) intrusives: Implications for crustal versus mantle origin

In a study of cogenetic zircons it was found that the measured Pb^(208)/Th^(232) and Pb^(207)/U^(235) ratios formed a linear array in the corresponding coupled Pb-U-Th evolution diagram, which has an upper intersection on the concordia curve at the same time point as that determined by the Pb^(206)/U^(238), Pb^(207)/U^(235) array. Although the U-Pb data lie in the accessible region for nonfractionating daughter loss, the Th-Pb results lie outside the corresponding region. The zircon concentrates analyzed were shown to be multiphase assemblages with variable U and Th contents and variable Th/U ratios, even within single crystals. The zircons contain local domains of high radioactivity which appear to be highly discordant. A relationship between discordance and the average concentration of U and Th in a sample is given. The degree of discordance increases with the concentration of U and Th and with the increasing Th/U ratio, causing preferential loss of Pb^(208) and the departure from the region accessible to a single phase without fractionation. The theoretical aspects of the (equation image) diagram from the viewpoint of single-phase and multiphase assemblages are discussed, and it is shown that the variability of the Th/U ratio is of fundamental importance in understanding the evolution of the Th-U-Pb system. The existence of these systematics in nature may provide an additional independent dating system and a further means of studying transport from natural systems.

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Systematics in the Pb208-Th232, Pb207-U235,, and Pb206-U238 Systems

The Gotthard massif in central Switzerland, part of the autochthonous basement of the Alps, has been metamorphosed during the Alpine orogeny. Structural studies of the metasedimentary hornblende rocks along the southern border reveal the existence of several mineral generations which were formed at different times and under different conditions. The sequence of formation can be deduced from their mutual intergrowth and orientation relative to the fabric of the rock. Steiger [1962] concluded that the Alpine orogeny in this region consisted of two distinct tectonic phases separated by a thermal phase. The main tectonic phase was the dragging along of the Gotthard massif by the overlying northward-moving nappes. Most of the pre-Alpine mineral assemblage was crushed and new minerals were formed which show preferential N-S orientation (direction of the nappe movements). The subsequent thermal metamorphism was linked with anatectic processes in the Lepontinic region south of the Gotthard massif. At this time porphyroblastic minerals of random orientation were formed along the southern margin of the massif. A weaker tectonic phase consisting of an E-W contraction caused N-S oriented small folds and wrinklings in mica-rich schists, and produced cross biotites. Hornblende was formed in pre-Alpine times as well as during both the main tectonic and the thermal phase of the Alpine orogeny. K-Ar ages were determined for 17 hornblendes. N-S oriented hornblendes show exclusively ages of 46 m.y.; random hornblendes yield ages of 23 to 30 m.y. Partially oriented hornblendes give apparent ages of 23 to 112 m.y. The hornblende ages are definitely higher than Rb-Sr ages of bioties (16 m.y.) from this region [Jager, 1962]. It is concluded that the N-S oriented hornblendes give a minimum age Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W contriction of proximately date the period (23 to 30 m.y.) of the anatexis in the Lepontinic region. The Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W constriction of the massif and the Lepontinic region. Partially oriented hornblendes cannot be related to any particular event. They may in part represent relics of pre-Alpine origin which suffered differential loss of argon during the Alpine metamorphism.

Rudolf H. Steiger

Papers

Nd-Sr isotopic and geochemical characteristics of the southern Adamello (northern Italy) intrusives: Implications for crustal versus mantle origin

Systematics in the Pb208-Th232, Pb207-U235,, and Pb206-U238 Systems

Dating of orogenic phases in the central Alps by K-Ar ages of hornblende