Determination Of Mesozoic Magmatic Activity Sequence And Its Geological Significance In Lengshuikeng Pb-Zn-Ag Orefield,North Wuyi Mountain Area

Lengshuikeng Pb-Zn-Ag Orefield is located in the south of the Qinzhou-Hangzhou Suture Zone between the Yangtzi and Cathysia Paleo-plates.It is a unique porphyry deposit in the North Wuyi Mesozoic Volcanic Belt.Previous zircon U-Pb geochronological worked on the ignimbrites,tuffs and intrusive rocks from the orefield,they were yielded conflicting ages of Late Jurassic and Early Early-Cretaceous.The above results lead to differences in the geological age of the volcanic rock series;moreover,the age of the porphyry invading the volcanic rock series in the orefield is consistent with the age of the volcanic rock,and even the problem that the porphyry age is greater than the age of the volcanic rock.Therefore,based on the research on the stratigraphic sequence and lithology and lithofacies characteristics of the volcanic rocks in the ore field,we choose three pieces of rhyolite samples LSK301,LSK302 and LSK401 in volcanic rock series,two pieces of welded tuff samples LSK701,LSK201,two pieces of tuff samples LSK601,LSK602 and one piece of mineral-bearing granite porphyry sample LSK501,two pieces of feldspar porphyritic granite samples LSK001,LSK002,two quartz syenite porphyry samples LSK101,LSK102 for SHRIMP zircon U-Pb dating.The results are as follows.(1)The age-weighted mean values of zircon 206Pb/238 U for three rhyolite samples LSK301,LSK302 and LSK401 are(139.5±1.1)Ma(N=17,MSWD=1.16),(137.8±1.3)Ma(N=13,MSWD=1.6)and(139.4 ± 1.2)Ma(N =16,MSWD=1.2),indicating that the geological age of rhyolite in Lengshuikeng Pb-Zn-Ag Orefield was early stage of Early Cretaceous.(2)In the 206Pb/238 U age data of 16 zircons from the welded tuff LSK701 sample,the 14 are aged 137~141 Ma,the weighted average age is(139.2±0.98)Ma(MSWD=0.96);the 206Pb/238 U age data of 22 zircons in the welded tuff LSK201 sample,the 206Pb/238 U age of 12 zircons data range from 150~164 Ma,the weighted mean age is(157.8±3.2)Ma(MSWD=1.5),the 206Pb/238 U age of 8 zircons data range from 131~145 Ma,and the weighted mean age is(135.0±3.2)Ma(MSWD=1.6).If the minimum age in the samples is taken as the lower limit of the the welded tuff formation age,then the geological age of the welded tuffs LSK701 and LSK201 is early stage of Early Cretaceous.(3)In the 206Pb/238 U age data of 17 zircon samples from the tuff LSK601 sample,the 14 zircons are aged 153~164 Ma,the weighted average age is(159.0±1.4)Ma(MSWD=1.6);the tuff LSK602 sample has 14 analysis points of 206Pb/238 U age data,The age of a zircon is(413.2±4.4)Ma,the age of 3 zircons is 246~230 Ma,the age of 5 zircons is 153~158 Ma,and the age of 5 zircons is 135~144 Ma.If the minimum age in the sample is the lower limit of the tuff formation age,the geological age of the tuff LSK601 belongs to Late Jurassic,and the geological age of LSK602 belongs to early stage of Early Cretaceous.(4)The age-weighted mean value of zircon 206Pb/238 U of the mineral-bearing granite porphyry LSK501 sample is(157.3±3.4)Ma(N=12,MSWD=1.26),indicating that the geological age belongs to Late Jurassic.(5)The age-weighted mean value of zircon 206Pb/238 U from the feldspar porphyritic granite LSK001 and LSK002 samples are(134.6±1.3)Ma(N=12,MSWD=0.24)and(135.1 Ma±1.2)Ma(N=13,MSWD=0.067),which indicate the geological age belongs to early stage of Early Cretaceous;the age-weighted mean values of zircon 206Pb/238 U from the quartz syenite porphyry LSK101 and LSK102 are(134.0±1.1)Ma(N=13,MSWD=0.51)and(134.5±1.1)Ma(N=12,MSWD=0.34),which indicate the geological age belongs to early stage of Early Cretaceous.Therefore,the zircon U-Pb age of the rhyolite indicates that the geological age of the volcanic rock series in Lengshuikeng Pb-Zn-Ag Orefield belongs to early stage of Early Cretaceous.the volcanic rocks in Lengshuikeng Pb-Zn-Ag Orefield belong to the early Cretaceous volcanic activity and are consistent with the formation of volcanic rocks in the North Wuyi Mesozoic Volcanic Belts.Based on the petrographic characteristics and source of pyroclastic rock,the welded tuffs LSK701,LSK201 and tuff LSK602 may be the products of the Plinia and Vulcanian style volcanic eruptions.The minimum age represents the age of formation of volcanic clastic rocks,while the tuff LSK601 may be the product of the Strombolian style volcanic eruption.Its minimum age is the zircon age of Late Jurassic granite porphyry,which does not represent volcanic clastic rocks.In this way,It can be explained a reasonable geological interpretation of the zircon U-Pb age from pyroclastic rocks.When dealing with age data for volcanic-invasive complexes,the age of the captured early stage of Early Cretaceous volcanic series zircons should be removed,followed by zircon weighted average age treatment,and compared with the age before screening,the MSWD of the sample is significantly smaller,indicating that the data is more realistic and reliable,and the weighted mean age is also smaller than the age before removed.Compared with the age of the volcanic rock series,the sequence of magmatic activity of the volcanic-intrusive complexes can be distinguished.In summary,the determination of Mesozoic magmatic activity sequence of Lengshuikeng Pb-Zn-Ag Orefield is as follows.the first phase forms ore-bearing granite porphyry,which occurred in Late Jurassic;the second phase magmatic activity The second phase of magmatism is divided into two stages.the first stage formed the volcanic series,which occurred in early stage of Early Cretaceous,there are different volcanic activity in in the orefield,forming a volcanic rock series with a small amount of felsic volcanic lava in the pyroclastic rock.The second stage formed sub-volcanic rocks such as quartz syenite porphyry and feldspar porphyritic granite,which occurred in early stage of Early Cretaceous,but later than volcanic rocks.Combined with previous knowledge,Lengshuikeng Pb-Zn-Ag Orefield can be divided into two mineralization events of Late Jurassic and Early Cretaceous.The two stages of metallogenic events are related to two porphyry intrusion activities.