<Curve>
    <Function>createCurve</Function>
    <Parameters>
        <Input>
            <BaseCurve>
                <CurveType Type="String">16fSmoothCurve</CurveType>
                <Ccy Type="String">USD</Ccy>
                <Tenors>
                     <Item Type="String">1M</Item>
                     <Item Type="String">3M</Item>
                     <Item Type="String">1U</Item>
                     <Item Type="String">Z1</Item>
                 </Tenors>
                <Rates>
                    <Item Type="String">.02123</Item>
                    <Item Type="String">.02214</Item>
                    <Item Type="String">.021234</Item>
                    <Item Type="String">.02674</Item>
                </Rates>
            </BaseCurve>
        </Input>
    </Parameters>
</Curve>

var root = new XElement("Curve",
    new XElement("Function", "createCurve"),
    new XElement("Parameters"), etc);

var tenors = root.XPathSelectElements("//Tenors/Item");
var rates = root.XPathSelectElements("//Rates/Item");

var firstTenor = root.XPathSelectElement("//Tenors/Item");
var firstRate = root.XPathSelectElement("//Rates/Item");