Neoarchean granitic rocks from the Jiamiao area of the Dabie orogen:Implications on the formation and early evolution of the Yangtze Craton

Archean rock exposures in the Yangtze Craton of South China are scarce and have been only studied in small-scale outcrops for understanding its early crustal evolution.Geological investigations have been carried out for three Neoarchean gneissic granitic plutons from the Dabie orogen in the northeastern margin of the Yangtze Craton.Zircon U-Pb geochronology constrains the emplacement of high-K monzogranite at 2645±30 Ma,high-K syenogranite at 2630±37 Ma and Na-rich granodiorite at 2497±29 Ma.These findings provide direct evidence for the existence of∼2.7-2.5 Ga rocks in the Dabie orogen,which significantly contribute to the distribution and evolution of Neoarchean basement rocks in the Yangtze Craton.Zircon crystals from the three granite samples giveεHf(t)values of−0.9 to 3.7,−4.2 to−0.4 and−4.7 to 0.1,respectively,corresponding to T_(DM2) ages of 3186-2909,3372-3141 and 3297-3005 Ma.The magmatic zircons have δ^(18)O values of 5.06±0.27‰to 5.79±0.30‰(average 5.59‰),6.14±0.24 to 7.35±0.26‰(average 6.87‰)and 5.95±0.17‰to 7.09±0.17‰(average 6.55‰),respectively.Considering the possible decrease of δ^(18)O value due to the lead loss during post-crystallization alteration,the primary δ^(18)O values might be substantially higher than those of the normal mantle zircon value(δ^(18)O=5.3±0.6‰).These isotopic data indicate that the Jiamiao Neoarchean granites were largely generated through reworking of Paleo-Mesoarchean basement rocks with a minor contribution of supracrustal material.Integrating our findings with previous results,we infer that the Archean-Paleoproterozoic complexes/terranes of the Yangtze Craton have distinct petrogenesis and the timings of the transition from Na-rich to high-K granites,crustal growth,and tectonothermal evolution before∼2.0 Ga.However,all of them underwent metamorphism at about 2.0 Ga.These observations also reveal that the Yangtze Craton might comprise several microcontinents that evolved individually and collided to form a unified craton at∼2.0 Ga,synchronous with the assembly of the Columbia supercontinent.However,the lack of relevant data from the southeastern Yangtze Craton may have a critical influence on this conclusion and should be solved in the future.

Biogenic gas generation effects on anthracite molecular structure and pore structure

This study carries out a simulated experiment of biogenic gas generation and studies the effects of gas generation on the pore structure and molecular structure of anthracite by mercury intrusion porosimetry,X-ray diffraction(XRD)and Fourier transform infrared spectroscopy(FT-IR).The results show that methanogenic bacteria can produce biogenic gas from anthracite.CO_(2) and CH4 are the main components of the generated biogas.After generation,some micropores(<10 nm)and transitional pores(10–100 nm)in the coal samples transform into large pores.In the high-pressure stage(pressure>100 MPa)of the mercury intrusion test,the specific surface area decreases by 19.79%compared with that of raw coal,and the pore volume increases by 7.25%in total.Microbial action on the molecular structure causes changes in the pore reconstruction.The FT-IR data show that the side chains and hydroxyl groups of the coal molecular structure in coal are easily metabolized by methanogenic bacteria and partially oxidized to form carboxylic acids.In addition,based on the XRD data,the aromatic lamellar structure in the coal is changed by microorganisms;it decreases in lateral size(La)and stacking thickness(Lc).This study enriches the theory of biogenic coalbed gas generation and provides a pathway for enhancing the permeability of high-rank coal reservoirs.