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Silicic Large Igneous Provinces 20 by Scott Bryan Silicic Large Igneous Provinces Email: [email protected] Large Igneous Provinces (LIPs) are the end-product of huge additions of magma to the continental crust both at Introduction the surface and at depth. Since the first categorisation of !"#$$ &'()#*+,* +-.+,/$0 +--&,1 *) )2# 2,-$#0*+,-&,1 )3 *"$ .)45 LIPs by Coffin & Eldholm (1994), it has been recognised /+,)4)16 )3 7+#1$ 81,$)20 9#).&,/$0 :7890; 0&,/$ *"$6 <$#$ 3&#0* that LIPs are more varied in form, age and character, and 3)#'+446 -$3&,$- =6 >)33&, ? @4-")4' :ABBC; +#$D A; *"$ #$/)1,&5 this includes the recognition of Silicic LIPs. Silicic LIPs *&), *"+* 34))- =+0+4* 4+.+0 "+.$ 1$,$#+446 =$$, $'(4+/$- +0 04)< are the largest accumulations of primary volcaniclastic +-.+,/&,1E &,34+*$- (+")$")$5*6($ 4+.+0E 3$- 3#)' #$4+*&.$46 4)<5 rocks at the Earth's surface with areal extents >0.1 Mkm2 #+*$ +,- 4),154&.$- $#2(*&),0 :$F1FE G$43 $* +4FE ABBHI !")#-+#0), ? and extrusive and subvolcanic intrusive volumes G$43E ABBJ;I K; *"+* +44 7890 /),*+&, .+#&)20 (#)()#*&),0 )3 '+3&/ *) >0.25 Mkm3. The Late Palaeozoic to Cenozoic Silicic LIP 0&4&/&/ /)'()0&*&), .)4/+,&/ +,- .)4/+,&/4+0*&/ #)/L0 #$34$/*&,1 + .+#&$*6 )3 .)4/+,&/ +,- 0$-&'$,*+#6 (#)/$00$0 +,- (+4+$)$,.&#),5 events are the best recognised and are similar in terms of '$,*0E <"&/" /+, =$ 20$- *) *$0* (#$-&/*$- (42'$5#$4+*$- ("$,)'5 their dimension, crustal setting, volcanic architecture and $,+ :0$$ #$.&$< &, GL&44&,1 $* +4FE KMMH;I +,- N; *"$ #$/)1,&*&), *"+* geochemistry. The Silicic LIPs typically form linear /),*&,$,*+4 7890 /+, ,)* ),46 "+.$ 02=0*+,*&+4 .)42'$0 :OAMC L'N; (>2000 km long) volcanic–plutonic belts along volcanic )3 0&4&/&/ &1,$)20 #)/LE =2* *"+* 0)'$ 78950/+4$ &1,$)20 (#).&,/$0 rifted margins or as failed continental rifts. Igneous com- +#$ /)'(#&0$- +4')0* $,*&#$46 =6 0&4&/&/ &1,$)20 #)/L :P#6+, $* +4FE positions are volumetrically silicic-dominant (>65 wt% KMMK;F G&4&/&/ &1,$)20 #)/L0 +#$ +, &,*$1#+4 (+#* )3 +44 /),*&,$,*+4 7890 SiO2), but generally show a range of igneous composi- 3#)' *"$ )4-$0* 9#$/+'=#&+, :$F1FE !<&0* ? Q#$,/"E ABJNI !")#,$ ? tions from basalt through to high-silica rhyolite. The rhy- !#$,-+44E KMMAI P4+L$ $* +4FE KMMC; *) *"$ 6)2,1$0* >$,)R)&/ $S+'5 olites show transitional within-plate to calc-alkaline or (4$0E +,- +#$ (+#*&/24+#46 (#$.+4$,* &, *"$ T$0)R)&/U>$,)R)&/ /),*&5 convergent margin geochemical signatures, whereas both ,$,*+4 ))- =+0+4* (#).&,/$0 +,- +4),1 .)4/+,&/ #&3*$- '+#1&,0F 8, *"$0$ 7890E 0&4&/&/ .)4/+,&/ +,- .)4/+,&/4+0*&/ #)/L0 /+, 3)#' 02=5 low- and high-Ti (>2 wt% TiO2) magma types are present in the coeval mafic igneous rocks as commonly observed 0*+,*&+4 (+#*0 )3 *"$ $#2(*&.$ 0*#+*&1#+("6 +,- #$(#$0$,* + 0&1,&/+,* /),*#&=2*&), *) *"$ *)*+4 '+1'+*&/ )2*(2* )3 + 789 :P#6+, $* +4FE in continental flood basalt provinces. Several Silicic LIPs KMMK;F !"$&# 0&1,&/+,/$E ")<$.$#E &, *$#'0 )3 +#$+4 $S*$,*E $#2(*&.$ form a pre-rift magmatic event along volcanic rifted mar- .)42'$E #$4+*&.$ *&'&,1 )3 $#2(*&),0 *) +,- ($*#)1$,$*&/ #$4+*&),0"&(0 gins that develop through a consistent temporal pattern of <&*" *"$ ))- =+0+4*0E +,- $,.&#),'$,*+4 +,- /4&'+*&/ &'(+/*0 -2#&,1 LIP magmatism followed by rifting, uplift and seafloor- 789 $'(4+/$'$,*E "+.$ 1$,$#+446 #$/$&.$- 4$00 +**$,*&), &, *"$ 4&*$#+5 spreading. The difference between the Silicic LIPs and *2#$ *"+, *"$ +00)/&+*$- ))- =+0+4* 4+.+0F 8* &0 )3*$, 2,-$#+((#$/&+*$- other continental mafic-dominated LIPs is largely due to *"+* *"$ 0/+4$ )3 0)'$ )3 *"$ &,-&.&-2+4 0&4&/&/ $#2(*&.$ 2,&*0 &, 7890 &0 &,/#$-&=46 .+0* :$F1FE &, *"$ 9+#+,V5@*$,-$L+ <"$#$ *"$ 4+#1$0* 2,&*0 different crustal settings. Phanerozoic Silicic LIPs are /).$# +#$+0 OMFA TL'K; +,- /+, =$ 4+#1$# *"+, *"$ +00)/&+*$- ))- restricted to continental margins that comprise fertile, =+0+4* 4+.+0E 02/" *"+* *"$6 +#$ +'),10* *"$ 4+#1$0* .)42'$ *$##$0*#&+4 hydrous lower crustal materials built up by Phanerozoic $#2(*&.$ 2,&*0 0) 3+# #$/)1,&R$- :OWMMM L'N XY@I T&4,$# $* +4FE ABBWI subduction. The role of hydrous crustal additions and T+#0" $* +4FE KMMAI @<+#* $* +4FE ABBJE KMMC;F underplate formed during previous episodes of subduction 8, +--&*&), *) #$(#$0$,*&,1 + 0&1,&3&/+,* &1,$)20 /)'(),$,* seem crucial in triggering widespread crustal partial &, /),*&,$,*+4 7890E 0&4&/&/5-)'&,+*$- &1,$)20 (#).&,/$0 "+.$ melting, and preventing a dominantly mafic surface =$$, #$/)1,&0$- &, *"$ 4+0* AM 6$+#0 <&*" $#2(*&.$ .)42'$0E +#$+4 $S*$,*0 +,- )*"$# +**#&=2*$0 *"+* +#$ /)'(+#+=4$ *) *")0$ )3 *"$ expression to LIP events along these palaeo- and active /),*&,$,*+4 34))- =+0+4* (#).&,/$0 +,- )*"$# 7890 :P#6+, ? @#,0*E continental margins. Silicic LIPs represent important tar- 02='&**$-;E =2* "+.$ 4)< (#)()#*&),0 )3 =+0+4* $S(#$00$- +* *"$ gets for precious metal mineralisation and host extensive 02#3+/$F P#6+, $* +4F :KMMK; -$3&,$- *"$ *$#' ZG&4&/&/ 7+#1$ epithermal Au–Ag fields. They have been the sites of many 81,$)20 9#).&,/$[ :G&4&/&/ 789; *) -$0/#&=$ *"$0$ .)4/+,)5(42*),&/ large volume (>1000 km3 dense rock equivalent) silicic (#).&,/$0 )3 <"&/" *"$ @+#46 >#$*+/$)20 .)4/+,&/ #&3*$- '+#1&, )3 explosive eruptions and important sources of ash and $+0*$#, \20*#+4&+ :]"&*02,-+6 &1,$)20 (#).&,/$I P#6+, $* +4FE ABBHI KMMM;E +,- *"$ ^2#+00&/ >"), \&L$ (#).&,/$ )3 G)2*" \'$#5 aerosol contributions to the stratosphere. Future studies &/+ +,- *"$ \,*+#/*&/ 9$,&,024+ :9+,L"2#0* ? Y+($4+EABBWI need to integrate their environmental effects with those 9+,L"2#0* $* +4FE ABBJI KMMMI Y&4$6 ? 7$+*E ABBB; +#$ *<) <$445 from similarly large volume flood basalt eruptions. L,)<, $S+'(4$0 :!+=4$ AI Q&12#$ A;F March 2007 21 Figure 1 Global distribution of Phanerozoic Silicic LIPs. (Modied from Bryan & Ernst, submitted). Inferred extents to the Chon Aike and Kennedy-Connors-Auburn provinces are shown by dashed lines. 4),1 *"$ Table 1 Catalogue of well-dened Phanerozoic Silicic LIPs, ordered in terms of minimum (240$0F extrusive volumes. From Bryan et al. (submitted). :! TL'N <+0 @#,0*E 0"+#$ $S/425 0&.$46 /),*&,$,*+4 +,- 0&*$- +4),1 '+#1&,0 +0 *"$6 +#$ (#)-2/$- =6 4+#1$50/+4$ 7890 (#$5 #&3*$- $S+'(4$0 (#$0$#.$- +,- *"$&# /"+#+/*$#&0*&/0 "+.$ =$$, 02'5 +,- ).$# Silicic LIPs /"+#+/*$#&0*&/0F TL'K +#$ TL'K 3)# -$ G$/),-E "21$ (#)4$ +#$ TL'N TL'N )3 /)'(#&0$ *#+,0&5 +,- #"6)4&*&/ &1,&'=#&*$ &0 *"$ -)'&,+,* :.)4/+,&/; 789 Figure 2 Formation of Silicic LIPs on volcanic rifted margins and the timing of '+,6 formation of oceanic crust. As with the continental ood basalt provinces, the Silicic $.&-$,/$ LIPs precede breakup and formation of oceanic crust. Compare with gure 3 of )3 Menzies et al. (2002). Episodes, Vol. 30, no. 1 22 a b c d Figure 3 Maps of the Silicic LIPs. \; \20*#+4&+, (4+*$ T+,&5 TL'N \(*&+,5 ABBH;F &, +#$ _+&,+ P; /),*&,$,*+4 TP7E T+#&$ >; /), @+#46 h+*/" +#$+ T&--4$ *&'$F T)-& +,- March 2007 23 Table 2 Comparison of the Whitsunday Silicic LIP with recent large-volume silicic volcanic provinces. =$ /#)005/2* =6 '+/ -6L$0 :\44$,E KMMMI Magma ux rate is averaged eruptive ux, based on known extrusive volumes for the provinces. P#6+, $* +4FE KMMM;E <"&/" '&##)#0 .$#*&/+4 /)'()0&*&),+4 *#$,-0 &, *"$ ")0* .)4/+,&/ 0*#+*&1#+("6F !"$ ')#$ #$/$,*46 3)#'$- G&4&/&/ 7890 +#$ $S($/*$- *) "+.$ 0&'&4+#46 $S*$,0&.$ '&- *) 2(($# /#20*+4 1#+,&*&/ =+*")4&*" 2,-$#(&,,&,10 +,- -6L$ 0<+#'0E +,- ')#$ '+/ &1,$)20 2,-$#(4+*$ +* 4)<$# /#20*+4 -$(*"0 :Q$##+#& $* +4FE KMMk;F !"$ G&4&/&/ 7890 +#$ *"$ 4+#1$0* +//2'24+*&),0 )3 (#&'+#6 .)4/+,&/4+0*&/ #)/L0E #$(#$0$,*&,1 *"$ $,*&#$ #+,1$ )3 3#+1'$,*+4 (#)-2/*0 -$()0&*$- -&#$/*46 =6 $S(4)0&.$ +,- $3320&.$ $#2(*&), :]"&*$ ? h)21"*),E KMMk;F Y"6)4&*&/ &1,&'=#&*$ 3)#'0 *"$ ).$#<"$4'&,1 (#)5 9#)*$#)5 ()#*&), )3 .)4/+,&/ #)/L &, +44 *"$ G&4&/&/ 7890E *6(&/+446 #$(#$5 '+6 0$,*&,1 OJMd )3 *"$ *)*+4 0*#+*&1#+("&/ *"&/L,$00 :Q&12#$ W;F $S+'5 \4*")21" #"6)4&*$ &1,&'=#&*$5-)'&,+*$-E *"$ $#2(*&.$ 0*#+*&1#+("6 8,-&+ )3 + G&4&/&/ 789 /+, =$ /),0&-$#$- +0 *"$ o,$1+*&.$ &'+1$p )3 *"+* 789 )3 *"$ /),*&,$,*+4 34))- =+0+4* (#).&,/$0 :Q&12#$ W;F P)*" "+.$ + Y+,1$5h&4*+=+ 4)<$# '),)*),)20 .)4/+,&/ 0$/*&), OA L' *"&/LE =$&,1 -)'&,+*$- _&4$0E =6 $&*"$# 34))- =+0+4*0 :/),*&,$,*+4 34))- =+0+4* (#).&,/$0; )# #"65 @+0*$#, )4&*$ &1,&'=#&*$0 :G&4&/&/ 7890;E +,- =)*" "+.$ +, 2(($#E OA L' _#+,&*$5Y"6)4&*$ *"&/L =&')-+4 0$/*&), <"$#$ =+0+4* +,- #"6)4&*$ +#$ &,*$#=$--$- :Q&12#$ W;F a(($# =&')-+4 0$/*&),0 +#$ <$445$S(#$00$- &, *"$ *"$ q$'$, 0$/*&), )3 *"$ \3#)5\#+=&+, 34))- =+0+4* (#).&,/$ :aL0*&,0 *"$ 9$+*$ $* +4FE KMMW; +,- *"$ ]"&*02,-+6 G&4&/&/ 789 :P#6+, $* +4FE KMMM;F !+2() #"6)4&*&/ *"$ :$F1FE ), >4+=+21"E +4FE ! (#$5 ]"&*02,-+6 +4FE +0($/*0F *&4*&,1 789 *"$ TL'N /),*&,$,*+4 $#2(*&.$ 789 4$+0* .)45 (#).&,/$ TL'N 0$-&'$,5 #$'+&,0 +,-
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