The Dalat zone in southern Vietnam comprises a Cretaceous Andean-type magmatic arc with voluminous granitoids and contemporary volcanic rocks. On the basis of petrographical and mineralogical studies, the granitoids were subdivided into three suites: Dinhquan, Deoca and Cana. Rocks of the Dinhquan suite are hornblende-biotite diorites, granodiorites and minor granites. The Cana suite encompasses mainly leucocratic biotite-bearing granites with scarce hornblende. The Deoca suite is made up of granodiorites, monzogranites and granites. Geochemically, the granitoids are of subalkaline affinity, belong to the high-K, calc-alkaline series, and most of them display typical features of I-type granites. This paper presents the new Rb-Sr mineral and U-Pb zircon and titanite age data for the granitoids, which establish the ages of the plutonic suites as: the Dinhquan at~112-100 Ma, Cana at~96-93 Ma and Deoca at~92-88 Ma. These ages are significantly different from earlier publications, and indicate that the earliest magmatism in the Dalat zone began at 112 Ma ago, that is~30-50 Ma later than previously thought. Our geochronological data are also support the continuation of an Andean-type arc running from SE China via southern Vietnam to SW Borneo.
Granites of the Thien Ke pluton that are exposed in the Tam Dao region are of sub-alkaline affinity, belong to the high-K calc-alkaline series, and display typical features of S-type granite. The rocks have high ASI values of 1.14-1.41 and are characterized by high initial 87 Sr/
86Sr ratios (0.7264-0.7270) and low ε Nd (456 Ma) values (−6.5 to −8.4). All of these characteristics, combined with~1.8 Ga T DM model ages and low Mg# of 13-22, suggest an origin in the partial melting of crustal source rocks. LA-ICP-MS zircon U-Pb analytical results from two samples revealed emplacement ages of the granite at 456 ± 28 and 449 ± 12 Ma. These ages are entirely different from those reported in earlier publications and indicate the existence of a Caledonian tectonothermal event in the study area. The Nd-Sr isotopic data, in conjunction with inherited zircon ages (721-1876 Ma), imply Paleoproterozoic to Neoproterozoic crustal material as the major components of the underlying basement. The new geochronological data provide evidence for early Paleozoic magmatism in the Tam Dao region, which has not been previously identified.
We analyzed the agricultural land-use changes in the coastal areas of Tien Hai district, Thai Binh province, in 2005, 2010, 2015, and 2020, using Landsat 5 and Landsat 8 data. We used the object-oriented classification method with the maximum likelihood algorithm to classify six types of land uses. The series of land-use maps we produced had an overall accuracy of more than 80%. We then conducted a spatial analysis of the 5-year land-use change using ArcGIS software. In addition, we surveyed 150 farm households using a structured questionnaire regarding the impacts of climate change on agricultural productivity and land uses, as well as farmers’ adaptation and responses. The results showed that from 2005 to 2020, cropland decreased, while aquaculture land and forest land increased. We observed that the most remarkable decreases were in the area of rice (485.58 ha), the area of perennial crops (109.7 ha), and the area of non-agricultural land (747.35 ha). The area of land used for aquaculture and forest increased by 566.88 ha and 772.60 ha, respectively. We found that the manifestations of climate change, such as extreme weather events, saltwater intrusion, drought, and floods, have had a profound impact on agricultural production and land uses in the district, especially for annual crops and aquaculture. The results provide useful information for state authorities to design land-management strategies and solutions that are economic and effective in adapting to climate change.
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