Cenozoic magmatism in the Alps with special reference to the Ligurian knot

<p>More than half a century of investigations on the chemical and isotopic compositions and on geochronological data of the Cenozoic magmatic rocks in the Alps and the transition to the Apennine will be summarized. The Alps itself are dominated by a calc-alkaline series between ~42 and 30 Ma, which we summarized as Periadriatic magmatism. This magmatism includes also eroded volcanic parts and several dykes in the Southern Alps and Tyrol. In addition, Sesia Zone magmatic rocks are characterized by ultrapotassic, shoshonitic and calc-alkaline rocks between 33 and 30 Ma. Two other magmatic provinces are located in between the Alps and the Apennine: (1) Veneto volcanic province (=VVP; nephelinites, basanites and alkali basalts between 52 and 30 Ma); (2) Mortara volcano (~28 Ma). Another group is the Esterél magmatic province, which is located in the Alps and their direct foreland, but are not related to Alpine geodynamics. These are basalts, andesites and dacites with mantle signature developed between 40 and 20 Ma. In the hanging plate of the early Apennine geometry, some minor volcanic activity is preserved in Sardinia. The major volume of Apennine magmatism itself (Elba etc.) is Late Miocene-Pleistocene in age and is related to roll back dynamics of the Apennine.</p><p>The Eocene/Oligocene Periadriatic magmatism of the Alps requires significant melt production in the crust combined with some ACF processes. This is possible by infiltration of fluids in the mantle wedge and the lower crust and a change of P-T conditions in the mantle. Their calc-alkaline character is related to Na-dominated input in the mantle and crust, which is commonly inferred to result from subduction of oceanic units. Ultrapotassic melts in the Sesia-unit most likely result from infiltration of K-dominated fluids, related to dehydration of continental material. The dynamics of Apennine and possible related forearc extension would allow an extensional related magmatism in the Esterél. This magmatism overlap in time with Alpine magmatism, and require a small-scale mantle dynamic due to the development of two slabs. In addition, the VVP and the Mortara volcano are located on the non deformed continental fragment of Adria between the Alps and Apennine. This area is characterized by overfilled basins and local magmatism inside the Adriatic continental plate.</p><p>The sometimes minor preserved volumes, but well constrain timing of magmatic rocks at the interaction between Alps and Apennine give insights in the lower crust/mantle dynamics at Oligocene/Early Miocene times. These interpretations may differ from models based on upper crustal tectonics, due to the decoupling between upper crust and lower crust/mantle. </p>

REFLECTION OF MAGMATISM AND TECTONICS IN THE GEOPHYSICAL FIELDS OF THE SOUTHWESTERN PRIMORYE

The results of a comprehensive interpretation of geophysical fields (magnetic, gravity and radioactivity) of the South-Western Primorye (Khasansky district) are presented. The authors give schemes reflecting the relationship of geophysical fields with various surface and deep geological structures, and discontinuous disturbances of various orders. The deep boundaries of the local granitoids massifs of the Jurassic age emerging on the surface are determined. The distribution areas of Cenozoic magmatism are outlined. The anomalous zone of increased values of the radioactive field, associated with the introduction of magmatic bodies with a high content of potassium, was identified. An assumption is made about the existence of a long-lived fluid-permeable zone that controls magmatic processes in the region.

Post-collisional crustal thickening and plateau uplift of southern Tibet: Insights from Cenozoic magmatism in the Wuyu area of the eastern Lhasa block

The nature and timing of post-collisional crustal thickening and its link to surface uplift in the eastern Lhasa block of the southern Tibetan plateau remain controversial. Here we report on Cenozoic magmatism in the Wuyu area of the eastern Lhasa block. The Eocene (ca. 46 Ma) trachyandesites and trachydacites show slight fractionation of rare earth elements (REE), slightly negative Eu and Sr anomalies, and relatively homogeneous Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7050−0.7063, εNd(t) = −0.92 to −0.03, εHf(t) = +2.6 to +4.8). Previous studies have suggested Neo-Tethys oceanic slab break-off at 50−45 Ma; thus, the Wuyu Eocene magmatism could represent a magmatic response to this slab break-off and originate from relatively juvenile Lhasa crust. The Miocene (ca. 15−12 Ma) dacites and rhyolites have adakitic affinities, e.g., high Sr (average 588 ppm), Sr/Y (29−136), and La/Yb (30−76) values, low Y (4−12 ppm) and Yb (0.4−0.9 ppm) contents, and variable Sr-Nd and zircon Hf isotopes (87Sr/86Sr(i) = 0.7064−0.7142, εNd(t) = −11.7 to −3.7, εHf(t) = −3.2 to +4.5). Their more enriched Sr-Nd-Hf isotopes relative to the Eocene lavas indicate that they should be derived from mixed Lhasa lower crust comprising juvenile crust, ultrapotassic rocks, and probably Indian lower crust-derived rocks. This study has also revealed the transformation from Eocene juvenile and thin crust with a thickness of <40 km to Miocene mixed and thickened crust with a thickness of >50 km. Combined with published tectonic data, we suggest that both lithospheric shortening and magma underplating contributed to eastern Lhasa block post-collisional crustal thickening. Given the spatial-temporal distribution of eastern Lhasa block magmatism and regional geology, we invoke a post-collisional tectonic model of steep subduction of the Indian plate and subsequent westward-propagating plate break-off beneath the eastern Lhasa block, which caused the surface uplift.

Supplemental Material: Post-collisional crustal thickening and plateau uplift of southern Tibet: Insights from Cenozoic magmatism in the Wuyu area of the eastern Lhasa block

The analytical methods and results for the Wuyu Cenozoic magmatic rocks from the eastern Lhasa block of southern Tibet.

Supplemental Material: Post-collisional crustal thickening and plateau uplift of southern Tibet: Insights from Cenozoic magmatism in the Wuyu area of the eastern Lhasa block

The analytical methods and results for the Wuyu Cenozoic magmatic rocks from the eastern Lhasa block of southern Tibet.