Joseph G. Arth E. J. Moll-Stalcup 1991 <p><span>The Blackburn Hills volcanic field is one of several Late Cretaceous and early Tertiary (75–50 Ma) volcanic fields in western Alaska that comprise a vast magmatic province extending from the Arctic Circle to Bristol Bay. It consists of andesite flows, rhyolite domes, a central granodiorite to quartz monzonite pluton, and small intrusive rhyolite porphyries, overlain by basalt and alkali-rhyolites. Most of the field consists of andesite flows which can be divided into two groups on the basis of elemental and isotopic composition: a group having lower (</span><span class="math"><span id="MathJax-Element-1-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext></math>"><span class="MJX_Assistive_MathML">⁸⁷Sr⁸⁶Sr</span></span></span><span>)</span>_i<span>, higher (</span><span class="math"><span id="MathJax-Element-2-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext></math>"><span class="MJX_Assistive_MathML">¹⁴³Nd¹⁴⁴Nd</span></span></span><span>)</span>_i<span>, and moderate LREE and HREE contents (group 1), and a group having higher (</span><span class="math"><span id="MathJax-Element-3-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext></math>"><span class="MJX_Assistive_MathML">⁸⁷Sr⁸⁶Sr</span></span></span><span>)</span>_i<span>, lower (</span><span class="math"><span id="MathJax-Element-4-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>143</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Sr</mtext></math>"><span class="MJX_Assistive_MathML">¹⁴³Sr¹⁴⁴Sr</span></span></span><span>)</span>_i<span>, and lower HREE contents. Basalts are restricted to the top of the stratigraphic section, comprise the most primitive part of group 1&nbsp;</span><span class="math"><span id="MathJax-Element-5-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mtext>[(</mtext><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext><mtext>)</mtext><msub><mi></mi><mn>i</mn></msub><mtext>= 0.7033; (</mtext><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext><mtext>)</mtext><msub><mi></mi><mn>i</mn></msub><mtext>= 0.5129]</mtext></math>"><span class="MJX_Assistive_MathML">[(⁸⁷Sr⁸⁶Sr)i= 0.7033; (¹⁴³Nd¹⁴⁴Nd)i= 0.5129]</span></span></span><span>, and have trace-element ratios that are similar to those of oceanic island basalts (OIBs). In contrast to the basalts, group 1 andesites have higher (</span><span class="math"><span id="MathJax-Element-6-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext></math>"><span class="MJX_Assistive_MathML">⁸⁷Sr⁸⁶Sr</span></span></span><span>)</span>_i<span>&nbsp;and lower (</span><span class="math"><span id="MathJax-Element-7-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext></math>"><span class="MJX_Assistive_MathML">¹⁴³Nd¹⁴⁴Nd</span></span></span><span>)</span>_i<span>, and represent interaction of mantle-derived magmas with the lower crust of Koyukuk terrane. Group 2 andesites have (</span><span class="math"><span id="MathJax-Element-8-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext></math>"><span class="MJX_Assistive_MathML">⁸⁷Sr⁸⁶Sr</span></span></span><span>)</span>_i<span>&nbsp;and (</span><span class="math"><span id="MathJax-Element-9-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext></math>"><span class="MJX_Assistive_MathML">¹⁴³Nd¹⁴⁴Nd</span></span></span><span>)</span>_i<span>&nbsp;that are near bulk-earth values and probably formed by partial melting of the lower crust of Koyukuk terrane. The central pluton and rhyolite porphyries are isotopically uniform&nbsp;</span><span class="math"><span id="MathJax-Element-10-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mtext>(</mtext><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext><mtext>)</mtext></math>"><span class="MJX_Assistive_MathML">(⁸⁷Sr⁸⁶Sr)</span></span></span>_i<span>&nbsp;≈ 0.704, (</span><span class="math"><span id="MathJax-Element-11-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext></math>"><span class="MJX_Assistive_MathML">¹⁴³Nd¹⁴⁴Nd</span></span></span><span>)</span>_i<span>&nbsp;≈ 0.51275, and are interpreted to have formed by melting of young mafic to intermediate crustal rocks or by fractionation of group 1 andesites. The rhyolite domes have an isotopic range similar to that of the basalts and andesites&nbsp;</span><span class="math"><span id="MathJax-Element-12-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><mtext>[(</mtext><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext><mtext>)</mtext><msub><mi></mi><mn>i</mn></msub><mtext>= 0.70355-0.70499; (</mtext><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext><mtext>)</mtext><msub><mi></mi><mn>i</mn></msub><mtext>= 0.51263&amp;#x2013;0.51292]</mtext></math>"><span class="MJX_Assistive_MathML">[(⁸⁷Sr⁸⁶Sr)i= 0.70355-0.70499; (¹⁴³Nd¹⁴⁴Nd)i= 0.51263–0.51292]</span></span></span><span>, which suggests they formed by fractionation of the and site and basalt magmas. Although some workers have suggested that the volcanic field is underlain by old continental crust, none of the data require the presence of Paleozoic or Precambrian continental middle or upper crust under this part of the volcanic field. However, the ultimate source of some of the rocks in the Yukon-Koyukuk province that have high&nbsp;</span><span class="math"><span id="MathJax-Element-13-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>87</mn></msup><mtext>Sr</mtext><msup><mi></mi><mn>86</mn></msup><mtext>Sr</mtext></math>"><span class="MJX_Assistive_MathML">⁸⁷Sr⁸⁶Sr</span></span></span><span>&nbsp;and low&nbsp;</span><span class="math"><span id="MathJax-Element-14-Frame" class="MathJax_SVG" data-mathml="<math xmlns=&quot;http://www.w3.org/1998/Math/MathML&quot;><msup><mi></mi><mn>143</mn></msup><mtext>Nd</mtext><msup><mi></mi><mn>144</mn></msup><mtext>Nd</mtext></math>"><span class="MJX_Assistive_MathML">¹⁴³Nd¹⁴⁴Nd</span></span></span><span>&nbsp;ratios may be old sub-continental mantle and/or lower crust, which was previously subducted beneath the Yukon-Koyukuk province during Early Cretaceous arc-continent collision.</span></p> application/pdf 10.1016/0016-7037(91)90073-E en Elsevier Isotopic and chemical constraints on the petrogenesis of Blackburn Hills volcanic field, western Alaska article