Tempo-spatial variation of the late Mesozoic volcanism in Southeast China testing the western Paleo-Pacific Plate subduction models

The westward subduction of Paleo-Pacific plate (PPP) played a governing role in tectonic evolution of East Asia. Although various PPP subduction models have been proposed, the subduction age and dynamical process of the PPP remain controversial. In this study, we investigate the geochronology of extrusive rocks and tempo-spatial variations of the late Mesozoic volcanism in Southeast China. We reported zircon U-Pb ages of new 48 extrusive rock samples in the Shi-Hang tectonic zone. Together with the published data, ages of ~ 300 rock samples from ~ 40 lithostratigraphic units were compiled, potentially documenting a relatively complete history and spatial distribution of the late Mesozoic volcanism in Southeast China. The results show that the extrusive rocks spanned ~ 95 Myr (177–82 Ma), but dominantly ~ 70 Myr (160–90 Ma), with two main age populations of 145–125 Ma and 105–95 Ma. We propose that these ages represent the intervals of the Yanshanian volcanism in Southeast China and the western subduction of the PPP, within which two intensive volcanic eruptional pulses happened. Spatially, the age geographic pattern of extrusive rocks is both the oldest and youngest age clusters occurring in the CZ and the younger intensive group in the SHTB, indicating that the late Mesozoic volcanism migrated northwestly from the coast to the inland prior to ~ 145 Ma and subsequently retreated southeastly back to the coast. This migration pattern is interpreted to result from a northwestward subduction followed by a southeastward rollback or retreat of the PPP.

PETROGRAPHIC CHARACTERISTICS OF MAFIC EXTRUSIVE ROCKS ALONG THE SOUTHWESTERN PART OF MAJEVICA

Total number of nineteen mafic extrusive rock samples is subjected to the detailed optical examination.Based on definition of mineral composition, structure, texture, type and intensity of alteration processes,and taking into consideration the recent classification schemes, different lithotypes are defined. Themost frequent are spilites distinguished by classical "spilite" mineralogy (albite), and very often with"quench" texture and amygdaloidal structure. Diabases consist of primary plagioclase andclinopyroxene which are often decomposed and disintegrated. Diabase rocks are mostly characterisedwith ophitic texture and massive structure. Coarse grained varieties are characterized as dolerites, therocks in which intensity of alteration process is significantly lower than in previous lithotypes. Varietieswith frequent occurrences of amphibole got adjective amphibolic. The most common and the mostintense alteration processes are albitization and chloritization. Alteration processes of lower intensityare actinolitization, pumpellytization, zeolitization, carbonation-calcitization, limonitization,kaolinitization and silicification. Depending on alteration degree a certain lithotypes are named withprefix meta (metadiabases and metadolerites).

The Stability and Management of “Sandwich” Type Slopes in Extrusive Rock Area

Northern area of Hainan Island is located in the south of Leiqiong faulted zone; new tectonic movement and volcanic activity have intense since late tertiary. Cenozoic volcano in the region is developed, various types of extrusive rocks widely distributed which often directly cumulate on the Cenozoic sedimentary soil layers, then covered by more loose soil, form a soft - hard - soft “sandwich” type slope with a poor stability. Based on the study of this slope and simulate its stability, this paper proposed this kind of slope's destruction mechanism as well as the management method

LOW‐AMPLITUDE AEROMAGNETIC ANOMALIES IN SOUTHEASTERN MISSOURI

This study shows that aeromagnetic anomalies of less than 200 gammas are associated with topographic relief of exposed Precambrian granitic and volcanic rocks of the St. François Mountains. Anomalies resulting from hills coarsely crystalline granite are as high as 100 gammas in amplitude, whereas anomalies over comparable hills of fine‐grained rocks, such as granophyre or devitrified volcanic rock, are as much as 200 gammas. Anomalies caused by normal faulting or shearing of igneous rocks have amplitudes of less than 100 gammas; they are observed best in profile. These anomalies are superposed on larger magnetic features related to pendants of volcanic rock in the roof of a granite batholith and are distinguished from large lateral variations in magnetic intensity by their low amplitude and small areal extent. Analyses of the compound anomalies yielded the subsurface configuration of isolated roof pendants of resistant extrusive rock in some areas. As many of these pendants have negligible remanent magnetization, induction theory was used in the analyses. Application of these results to the lead‐mining areas shows that aeromagnetic patterns of low amplitude can guide mineral exploration in the region flanking the Ozark uplift, where isolated, buried hills of Precambrian igneous rocks controlled the development of some lead‐bearing sedimentary structures in the overlying Cambrian carbonate strata.